Inclusion Criteria:  Has advanced, metastatic or unresectable GI tract tumors that are refractory or intolerant to existing available therapies and for whom the investigator recommends fluoropyrimidine monotherapy. Has measurable disease in accordance with Respond Evaluation Criteria in Solid Tumors (RECIST) guidelines (Version 1.1). Is aged ≥18 years Has not received treatment with intravenous (IV) 5 FU or oral 5 FU analogs in the 4 weeks preceding enrollment Has Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2 at study entry  Has adequate bone marrow, liver, and renal function as assessed by the following laboratory requirements conducted within 7 days before starting study treatment:  peripheral ANC of ≥1.5 × 109/L platelet count of ≥75 × 109/L without growth factor/transfusion hemoglobin ≥8.5 g/dL without growth factor/transfusion estimated glomerular filtration rate >50 mL/min total bilirubin <2 × upper limit of normal (ULN); <5 × ULN if patient has liver metastases, biliary tract cancer; or ≤3 × ULN if the patient has Gilbert's disease Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) <2.5 × ULN, with liver metastasis <5 × ULN international normalized ratio (INR) <1.5 Has a life expectancy of at least 12 weeks Female patients of childbearing potential and male patients with partners capable of reproduction must agree to use an effective contraceptive method from the time of Screening through 60 days after the last dose of capecitabine Females of childbearing potential must have a negative serum pregnancy β human chorionic gonadotropin pregnancy test result Willingly provides written, informed consent. Has resolution or stabilization of acute toxicity from prior therapy to Grade <2 - except Grade 2 neuropathy If patient has human immune deficiency virus (HIV) infection, it is controlled with undetectable viral load with antiretroviral treatment. If patient has hepatitis C infection and received antiviral treatment, has a negative viral load at Screening If patient has chronic hepatitis B infection and is receiving antiviral treatment, has a negative viral load at Screening. Is willing and able to comply with all protocol required visits and assessments  Exclusion Criteria:  Is unable to take oral medication or malabsorption syndromes potentially interfering with medication absorption (e.g., short bowel syndrome or chronic, partial bowel obstruction) Has history or presence of clinically significant abnormal 12 lead ECG results, in the investigator's opinion Has current brain metastasis Has prolonged QTc (with Fridericia's correction) of >480 msec in men and women performed at Screening Has a history of prolonged QTc interval, ventricular tachycardia/fibrillation or significant ventricular arrhythmia, or Torsades de Pointes, or a history of ventricular ablation for arrhythmia Has congenital long QT syndrome or a family history of long QT syndrome Has other clinically significant cardiac disease including, but not limited to, uncontrolled angina, myocardial ischemia or infarction within 6 months, congestive heart failure >Class II per the New York Heart Association, or history of myocarditis Has an electrolyte disturbance, such as uncorrected hypokalemia/hyperkalemia, hypomagnesemia, or hypocalcemia. Patients can be enrolled following successful correction of an electrolyte disturbance. Is currently using any drugs included in the prohibited medications list in the protocol (including those that can prolong QTc) that cannot be discontinued Has known hypersensitivity to any of the components of study treatments Has other primary cancer requiring treatment within the last 3 years, except for cervical intraepithelial neoplasia, ductal carcinoma in situ, or completely excised squamous or basal cell carcinoma Is a pregnant or lactating female Had major surgery, open biopsy, or significant traumatic injury within 4 weeks prior to the first dose of study treatment Is receiving or has received any investigational treatment within 4 weeks prior to study entry, or participating in another clinical study Has known DPD deficiency