Revolutionizing cancer treatment, immune checkpoint inhibitors (ICIs) have nonetheless encountered a challenge: a substantial portion of gastric cancer (GC) patients don’t respond to this therapy. This pressing issue necessitates a deeper understanding of the resistance mechanisms against ICIs and the identification of robust biomarkers that can predict patient response to ICIs from the outset of treatment. In our project, we collected GC tissue samples from 28 patients prior to their initiation of anti-programmed death 1 (PD-1) immunotherapy. Employing high-resolution mass spectrometry (MS), we conducted comprehensive protein quantification. We then proceeded to analyze the disparities in protein expression profiles, associated pathways, and the characteristics of the tumor microenvironment (TME) between responder and non-responder groups. Ultimately, our aim is to delve into the mechanisms that give rise to differential therapeutic efficacy and thereby uncover reliable biomarkers for predicting ICI response in GC patients.