Tadalafil Effect + Chemotherapy in Resectable Gastric/GEJ Cancer

The goal of this clinical trial is to assess the ability of Tadalafil alone and in combination with neoadjuvant FLOT (5-Fluorouracil, Oxaliplatin, and Docetaxel) chemotherapy to suppress myeloid derived suppressor cells (MDSCs) in patients with resectable gastric or gastroesophageal junction adenocarcinoma. Resectable means the tumor may be removed through surgical intervention. Neoadjuvant chemotherapy is chemotherapy received before the primary course of treatment i.e.surgical intervention. The main questions it aims to answer are: * Is Tadalafil treatment with FLOT feasible and safe? * How does tadalafil treatment with FLOT affect the tumor microenvironment (TME)? * Will 8 weeks of neoadjuvant exposure to tadalafil with chemotherapy reduce MDSCs in the TME? Participants will receive Tadalafil for 14 days followed by combination of Tadalafil + FLOT for approximately 8 weeks as a part of standard of care neoadjuvant treatment in the window between cancer diagnosis and surgical intervention to remove their tumor. Tumor tissue, blood, and urine will be collected at the start of the study, after 2 weeks of treatment with Tadalafil alone, and around the time of surgical intervention. Saliva will also be collected at the start of the study.