Inclusion Criteria:

          -  DOSE ESCALATION COHORT:

          -  Patients must have histologically confirmed solid tumor malignancy that is not curable
             with standard approaches. Gemcitabine must be considered a standard therapy for the
             participant's malignancy

          -  Patients must have a measurable disease, in at least one lesion, for both the dose
             escalation and expansion cohorts, as defined by Response Evaluation Criteria in Solid
             Tumors (RECIST) version (v)1.1

          -  Patients must have received one line of treatment for their incurable cancer before
             enrolling in this trial. Patients with rare malignancies for which there is no
             accepted standard chemotherapy regimen can enroll without any prior treatments

          -  Patients must not have received more than two lines of cytotoxic chemotherapy

               -  Patients can have received prior gemcitabine

               -  Adjuvant chemotherapy is counted as one line of treatment if patients received it
                  within 6 months of their cancer recurring

               -  There is no limit for lines of prior targeted therapies or immunotherapy

               -  Patients who received a prior PARP inhibitor must have had progressive disease,
                  or intolerable toxicity, on the PARP inhibitor prior to enrolling on the study

          -  DOSE EXPANSION COHORT:

          -  Participants must have a histologically confirmed advanced pancreatic adenocarcinoma
             or ovarian cancer (high grade serous ovarian, primary peritoneal or fallopian tube
             cancer) that is not curable with standard approaches. Patients with both metastatic
             pancreatic cancer and unresectable pancreatic cancer are eligible

          -  Ovarian cancer:

               -  Patients with ovarian cancer must have platinum-resistant disease, defined as
                  progression within 6 months after the last platinum regimen

               -  Patients with ovarian cancer cannot have received more than one prior regimen in
                  the platinum-resistance setting

          -  Pancreatic cancer:

               -  Patients with pancreatic cancer cannot have received more than one line of
                  cytotoxic chemotherapy in the metastatic setting

                    -  Adjuvant chemotherapy is not counted as one line of treatment, if patients
                       received it more than 6 months prior to their cancer recurring

               -  There is no limit for lines of prior targeted therapies, such as PARP inhibitors,
                  or immunotherapy

          -  Patients must have a biopsiable disease and at least one separate measurable lesion

          -  DOSE ESCALATION AND EXPANSION COHORTS:

          -  Eastern Cooperative Oncology Group (ECOG) performance status =< 2 (Karnofsky >= 60%)

          -  Leukocytes >= 3,000/mcL

          -  Hemoglobin >= 10 g/dL (no red blood cell transfusion is allowed within 3 weeks before
             starting the trial)

          -  Neutrophil count >= 1,500 K/mcL (participants must not have received colony
             stimulating factors [e.g., granulocyte colony-stimulating factor, granulocyte
             macrophage colony stimulating factor or recombinant erythropoietin] within 3 weeks
             before initiation of protocol therapy)

          -  Platelets >= 100,000 /mcL

          -  Albumin >= 2.8 mg/dL

          -  Total bilirubin =< 1.5 x institutional upper limit of normal (ULN)

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase [SGOT]) /
             alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase [SGPT]) =< 3 x
             institutional ULN

          -  Creatinine clearance =< 1.5 x institutional ULN OR glomerular filtration rate (GFR) >=
             50 mL/min/1.73 m^2

          -  Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral
             therapy with undetectable viral load within 6 months are eligible for this trial

          -  For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral
             load must be undetectable on suppressive therapy, if indicated

          -  Patients with a history of hepatitis C virus (HCV) infection must have been treated
             and cured. For patients with HCV infection who are currently on treatment, they are
             eligible if they have an undetectable HCV viral load

          -  Patients with treated brain metastases or primary brain tumors are eligible if
             follow-up brain imaging after central nervous system (CNS)-directed therapy shows no
             evidence of progression for >= 4 weeks after the last date of treatment are permitted,
             and if they are no longer taking corticosteroids for at least 4 weeks prior to
             beginning the protocol

          -  Patients with a prior or concurrent malignancy whose natural history or treatment does
             not have the potential to interfere with the safety or efficacy assessment of the
             investigational regimen are eligible for this trial with permission of the principle
             investigator of the trial

          -  Patients with known history or current clinically significant symptoms of cardiac
             disease, or history of treatment with cardiotoxic agents, should have a clinical risk
             assessment of cardiac function using the New York Heart Association functional
             classification. To be eligible for this trial, patients should be class 2B or better

          -  The effects of BAY 1895344 on the developing human fetus are unknown. For this reason
             and because DNA-damage response inhibitor agents as well as other therapeutic agents
             used in this trial are known to be teratogenic, women of child-bearing potential and
             men must agree to use adequate contraception (hormonal or barrier method of birth
             control; abstinence) prior to study entry and for the duration of study participation
             and for 6 months after completion of BAY 1895344 administration. Should a woman become
             pregnant or suspect she is pregnant while she or her partner is participating in this
             study, she should inform her treating physician immediately. Men treated or enrolled
             on this protocol must also agree to use adequate contraception prior to the study, for
             the duration of study participation, and 6 months after completion of BAY 1895344
             administration

          -  Ability to understand and the willingness to sign a written informed consent document

        Exclusion Criteria:

          -  Patients cannot receive chemotherapy, targeted therapy or immunotherapy within 3 weeks
             of study entry

          -  Patients who have had radiotherapy within 4 weeks prior to entering the study

          -  Patients who have not recovered from adverse events due to prior anti-cancer therapy
             (i.e., have residual toxicities > grade 1) with the exception of alopecia and
             lymphopenia

          -  Participants must not have received investigational therapy administered =< 4 weeks or
             within a time interval less than at least five half-lives of the investigational
             agent, whichever is longer, before initiation of protocol therapy

          -  Participants with known untreated brain metastases are excluded from this clinical
             trial

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to BAY 1895344 or gemcitabine

          -  Patients receiving any medications that are substrates of CYP3A4 with a narrow
             therapeutic window, or strong inhibitors/inducers of CYP3A4 are ineligible, if they
             cannot be transferred to alternative medication. Because the lists of these agents are
             constantly changing, it is important to regularly consult a frequently-updated medical
             reference. As part of the enrollment/informed consent procedures, the patient will be
             counseled on the risk of interactions with other agents, and what to do if new
             medications need to be prescribed or if the patient is considering a new
             over-the-counter medicine or herbal product

          -  Patients with uncontrolled intercurrent illness including but not limited to ongoing
             or active infection, symptomatic congestive heart failure, unstable angina pectoris,
             clinically significant cardiac arrhythmia, uncontrolled major seizure disorder,
             unstable spinal cord compression, superior vena cava syndrome or psychiatric
             illness/social situations that would limit compliance with study requirements are
             excluded

          -  Patients with psychiatric illness/social situations that would limit compliance with
             study requirements

          -  Pregnant women are excluded from this study because BAY 1895344 as a DNA-damage
             response inhibitor, and gemcitabine may have the potential for teratogenic or
             abortifacient effects. Because there is an unknown but potential risk for adverse
             events in nursing infants secondary to treatment of the mother with BAY 1895344
             breastfeeding should be discontinued if the mother is treated with BAY 1895344 and for
             4 months after end of treatment. These potential risks may also apply to other agents
             used in this study

          -  Patients who have successfully undergone treatment for another, unrelated clinically
             relevant cancer, >= 3 years post final treatment, are eligible to participate in this
             study

          -  Patients cannot have received radiation to more than 25% of their hematopoietically
             active bone marrow. Pelvic radiation is considered to affect 25% of the
             haematopoietically active bone marrow, and only one prior course of pelvic radiation
             is allowed (Hayman et al., 2011)

          -  Patients previously treated with an ATR inhibitor are excluded

          -  Participants who have undergone major surgery =< 4 weeks before initiating protocol
             therapy must have sufficiently recovered from adverse events caused by the procedure,
             as judged by the treating investigator

          -  Subjects with a gastrointestinal disorder or malabsorption that could potentially
             affect the absorption of the study drug are excluded

          -  Participants with a history of a clinically relevant second primary malignancy within
             the past 2 years are excluded. Exceptions include resected basal and squamous cell
             carcinomas of the skin and completely resected carcinoma in situ of any type

          -  Patients not able to swallow tablets

          -  For the Dose Expansion Cohort, patients who cannot safely undergo tumor biopsies are
             excluded