Neratinib In Combination With Chemotherapy/Trastuzumab/Pembrolizumab In HER2 Gastroesophageal Cancer

All participants will take 240mg neratinib by mouth every day beginning cycle 1 day 1.

All participants will receive standard dosing (6mg/kg loading dose, 4 mg/kg subsequent doses) of Trastuzumab by IV infusion day 1 of each 2 week cycle.

All participants will receive 85mg Oxaliplatin by IV infusion day 1 of each 2 week cycle.

All participants will receive 400 mg/m\^2 5-Fluorouracil (5FU)+ 400 mg/m\^2 leucovorin day 1 of each 2 week cycle; 5FU continuous infusions on days 1 and 2 of each 2 week cycle. (5FU + leucovorin may be eliminated from regimen per PI discretion)

All participants will receive 400mg Pembrolizumab by IV infusion day 1 of every 3rd 2 week cycle.