Vedolizumab for the Treatment of Collagenous Gastritis

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

Inclusion Criteria: * Age 14 or older * Weight of at least 40kg * Has a biopsy-confirmed diagnosis of collagenous gastritis * Agrees to all required study procedures * Presence of one or more collagenous gastritis symptom at a severity of Grade 1 or higher at the time of enrollment: abdominal pain, nausea, diarrhea, bloating, vomiting, flatulence, constipation, fatigue, headache, rash Exclusion Criteria: * Unable to independently provide informed consent or assent * Allergic to vedolizumab or any of its components