Genetic Analysis-Guided Irinotecan Hydrochloride Dosing of mFOLFIRINOX in Treating Patients With Locally Advanced Gastroesophageal or Stomach Cancer

Inclusion Criteria: * Histologically confirmed locally advanced gastric (primary endpoint includes proximal and mid-body stomach) or esophagogastric adenocarcinoma; distal gastric (antral) adenocarcinomas are eligible for enrolment but will not be included in the primary analysis * Locally advanced disease as determined by endoscopic ultrasound (EUS) stage \> primary tumor (T) 3 and/or any T, lymph nodes (N)+ disease without metastatic disease (Mx) * All patients must have diagnostic laparoscopy with diagnostic washings for cytology; both cytology positive and negative patients are eligible for enrolment, but only cytology negative patients will be included in the primary analyses; gross peritoneal disease is not eligible * Eastern Cooperative Oncology Group (ECOG) performance status =\< 1 * Eligible for surgery with curative intent * Absolute neutrophil count (ANC) \>= 1250/ul * Hemoglobin \>= 9 g/dL * Platelets \>= 100,000/ul * Total bilirubin \< 1.5 x upper limit of normal * Serum glutamic oxaloacetic transaminase (SGOT) and serum glutamate pyruvate transaminase (SGPT) \< 2.5 x upper limit of normal for patients without liver metastases OR SGOT and SGPT \< 5 x upper limit of normal for patients with liver metastases * Creatinine =\< 1.5 x upper limit of normal * Measurable or non-measurable disease by Response Evaluation Criteria in Solid Tumor (RECIST) 1.1 will be allowed * Women of child-bearing potential and men must agree to use adequate contraception (hormonal or bar…