Background: The PD-L1 IHC 22C3 pharmDx used on the Dako Autostainer Link 48 (ASL48) staining platform is an established method for assessing programmed death-ligand 1 (PD-L1) expression in tumor tissue and determining patient eligibility for pembrolizumab treatment; however, the availability of this platform is limited in Europe and Asia. Objectives: The aims of this study were to develop and optimize protocols for the PD-L1 22C3 antibody concentrate with multiple immunohistochemistry staining platforms and to validate these protocols using PD-L1 combined positive score (CPS) with a cut-off of ≥ 1 in gastric or gastroesophageal junction adenocarcinoma. Design: The 22C3 antibody concentrate was tested and optimized protocols were developed for use with three staining platforms: Dako ASL48, Ventana BenchMark ULTRA, and Leica BOND-MAX. Tumor specimens (N = 120) from patients with gastric or gastroesophageal junction adenocarcinoma were used for the validation study; these specimens were evaluated independently by three pathologists for PD-L1 CPS as a continuous variable and using a cut-off of ≥ 1. PD-L1 IHC 22C3 pharmDx used on the Dako ASL48 platform served as the reference or gold standard. Results: The intraclass correlation coefficient of CPS as a continuous variable between the gold standard and each staining platform assessed was 0.910–0.989. When CPS was dichotomized based on a cut-off of ≥ 1, depending on the pathologist and the platform used, positive percentage agreement was 81–99% and negative percentage agreement was 90–100%. Interobserver agreement using the gold standard showed substantial agreement (κ = 0.779). Conclusion: The PD-L1 22C3 antibody concentrate can potentially be used with the laboratory-developed test on three commercially available immunohistochemistry staining platforms to determine PD-L1 expression in tumor samples from patients with gastric or gastroesophageal junction adenocarcinoma.