Inclusion Criteria:

          -  Age 18 years or older.

          -  ECOG performance status 0-2.

          -  Sign informed consent within 8 months after curative surgery and completion of
             standard of care perioperative and/or adjuvant therapy.

          -  HER2+ esophageal, GEJ, or gastric adenocarcinoma biopsy or resection specimen as
             defined by local HER2 IHC3+ or IHC 2+/FISH>2.0 expression.

          -  Must have genetic testing of DNA from primary tumor for somatic genomic alterations
             across a minimum of 50 genes.

          -  Must have undergone a complete curative surgical resection (R0).

          -  Must have completed standard of care (SOC) surgery, neoadjuvant or adjuvant therapy

          -  CtDNA will be tested at a minimum of four weeks after completion of surgery and
             standard perioperative and/or adjuvant therapy. To be eligible for
             trastuzumab/pembrolizumab or trastuzumab therapy, the patients must have positive
             ctDNA (as defined in section 7.0) within 8 months after completion of appropriate
             standard of care therapy (surgery, chemotherapy, radiation as appropriate). For all
             patients, if the initial ctDNA has a negative result, ctDNA can be re-tested 4 weeks
             later, within 9 months of completion of standard therapy.

          -  Demonstrate adequate organ function as defined in Table 1.

        Hematological Absolute neutrophil Count (ANC): ≥ 1,500 /mcL Platelets: ≥ 100,000 /mcL
        Hemoglobin: ≥ 9 g/dL

        Renal Serum creatinine ≤ 1.5 X upper limit of normal (ULN)

        Hepatic Serum total bilirubin ≤ 1.5 X ULN OR Direct bilirubin ≤ ULN for subjects with total
        bilirubin levels > 1.5 ULN. Except patients with Gilbert's disease (≤ 3 x ULN) AST and ALT
        ≤ 2.5 X ULN Albumin ≥ 3 mg/dL

        Exclusion Criteria:

          -  Patients who have not recovered from serious adverse events (as determined by treating
             MD) related to surgery.

          -  Presence of metastatic or recurrent disease.

          -  Had R1 (microscopic residual tumor) or R2 resection (macroscopic residual tumor at
             resection margin).

          -  Left ventricular ejection fraction <50% within 1 month of screening by MUGA or
             echocardiogram.

          -  Is currently participating and receiving study therapy or has participated in a study
             of an investigational agent and received study therapy or used an investigational
             device within 4 weeks of the first dose of treatment.

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to the first dose of trial
             treatment.

          -  Patients who have received acute, low dose, systemic immunosuppressant medications
             (e.g., dexamethasone containing antiemetic regimen or steroids as CT scan contrast
             premedication) may be enrolled.

          -  The use of inhaled corticosteroids and mineralocorticoids (e.g., fludrocortisone) for
             patients with orthostatic hypotension or adrenocortical insufficiency is allowed.

          -  Has a known history of active TB (Bacillus tuberculosis)

          -  Hypersensitivity to pembrolizumab or any of its excipients.

          -  Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy
             within 2 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at
             baseline) from adverse events due to a previously administered agent.

             ° Note: If subject received major surgery, they must have recovered adequately from
             the toxicity and/or complications from the intervention prior to starting therapy.

          -  Has known active central nervous system (CNS) metastases and/or carcinomatous
             meningitis

          -  Has known history of, or any evidence of active, non-infectious pneumonitis.

          -  Has an active infection requiring systemic therapy.

          -  Has a history or current evidence of any condition, therapy, or laboratory abnormality
             that might confound the results of the trial, interfere with the subject's
             participation for the full duration of the trial, or is not in the best interest of
             the subject to participate, in the opinion of the treating investigator.

          -  Has known psychiatric or substance abuse disorders that would interfere with
             cooperation with the requirements of the trial.

          -  Is pregnant or breastfeeding, or expecting to conceive or father children within the
             projected duration of the trial, starting with the pre-screening or screening visit
             through 120 days after the last dose of trial treatment.

          -  Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-CTLA-4 agent.

          -  Active or prior documented autoimmune or inflammatory disorders (including
             inflammatory bowel disease; systemic lupus erythematosus; Wegener syndrome
             [granulomatosis with polyangiitis]; myasthenia gravis; Graves' disease; rheumatoid
             arthritis, hypophysitis, uveitis) within the past 3 years prior to the start of
             treatment. The following are exceptions to this criterion:

               -  Subjects with vitiligo or alopecia

               -  Subjects with hypothyroidism (e.g., following Hashimoto syndrome) stable on
                  hormone replacement or psoriasis not requiring systemic treatment.

          -  Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies).

          -  Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA
             [qualitative] is detected).

          -  Has received a live vaccine within 30 days of planned start of study therapy.

             ° Note: Seasonal influenza vaccines for injection are generally inactivated flu
             vaccines and are allowed; however intranasal influenza vaccines (e.g., Flu-Mist®) are
             live attenuated vaccines, and are not allowed.

          -  Is unwilling to give written informed consent, unwilling to participate, or unable to
             comply with the protocol for the duration of the study.