Background: Gastroesophageal cancer (GEC), is a significant global health concern with a 5-year relative survival of 7% for metastatic GEC. Standard treatments often include surgery, chemotherapy, and radiation therapy, but their effectiveness can be limited. Chemotherapy is frequently utilized in the treatment of GEC, although no biomarkers for chemotherapy are used in the selection of the chemo agent. There are several well-known biomarkers of resistance and sensitivity for various chemo agents. We used targeted proteomics to quantify protein expression levels of several protein biomarkers associated with chemotherapy efficiency in 125 GEC patients. These include resistance markers for platinum agents (ERCC1), and tubulin inhibitors (TUBB3). It also includes sensitivity markers for irinotecan/topotecan (Topo1), and doxorubicin/etoposide (Topo2A). We also assessed the protein levels of several antibodies-drug conjugate markers (EGFR, Her2, Trop2, FR-alpha, Mesothelin, Axl, CLDN18.2, and Nectin-4). Methods: FFPE tumor tissues from 125 clinical GC patients were microdissected and solubilized for mass spectrometry-based targeted proteomic analysis in our CLIA-certified laboratory. 72 proteins were quantified from 2-3 sections of FFPE tissue. Results: 5-Fluorouracil (5-FU), taxanes, and platinum agents (such as carboplatin) are among the chemotherapy agents that are frequently used to treat GEC. TYMP, a sensitivity marker for 5-FU, was expressed in 32% of GEC cases. ERCC1, a resistance marker for platinum drugs, was expressed at 34%. Additionally, 48% of GEC exhibited TUBB3, a resistance marker for paclitaxel and docetaxel. Topoisomerase 1 (Topo1) is the target for irinotecan or topotecan, and is also the target for some ADCs (e.g. trastuzumab deruxtecan, sacituzumab govitecan etc.). Topo1 was expressed in 92% of GEC with a 10x range (415 - 4195 amol/μg). Another topoisomerase, Topo2A, targetable by etoposide, epirubicin, and doxorubicin, was expressed in 59% of GEC with 19x range (402-7804 amol/μg). ADC targets, such as EGFR (82% detected with 184x range of distribution), Her3 (18%, 4x), AXL (18%,7x), MET (27%, 19x), and Trop2 (50%, 57x) showed a wide range of protein expression. With a 56x range (318 - 17786 amol/μg), Her2 was found in 46% of GEC, of which 12% of GEC had overexpression of Her2 (>740 amol/μg), suggesting that the majority of GEC has low levels of Her2 (318-690 amol/μg), which is potentially targetable by newer anti-Her2 agents. Conclusions: Every cancer is different, and the ability to quantitate simultaneously 72 protein biomarkers from 2-3 FFPE sections provides a wealth of actionable information that, when combined with other multi-omics biomarker data, will help guide clinical treatment or patient stratification for clinical trials.