The goal of this clinical trial is to learn about how vedolizumab may affect patients with collagenous gastritis (CG). The main questions it aims to answer are: * Whether vedolizumab can reduce CG symptoms * Whether vedolizumab is safe to take for patients with CG Participants in this study will: * Receive vedolizumab through an IV ("infusion") * Complete a survey at each infusion visit * Have blood collected at each infusion visit * Undergo an endoscopy with biopsy at 2 timepoints
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Change in symtpoms
Timeframe: 12 Weeks