Background: Biomarkers have become increasingly important in the treatment of metastatic gastroesophageal cancers (mGEC) with new targeted and immune-based therapies relying on biomarkers to identify patients who may benefit from these therapies. Here, we aimed to identify real-world challenges mGEC care teams face in implementing biomarker testing and evidence-based treatment choices. Methods: In July–September 2022, 74 health care professionals (HCPs) who treat mGEC at 3 US community oncology clinics completed surveys assessing practice patterns, challenges, and confidence related to treating mGEC. HCPs (N = 24) participated in interdisciplinary audit and feedback (AF) sessions to examine clinical evidence and develop clinic-specific action plans to address identified gaps. Results: Participating practices reported that ~22% of their practice is dedicated to GEC care. Participants represented the interprofessional GEC care team: medical oncologists (27%), radiation oncologists (1%), surgeons (1%), advanced practice professionals (32%), nurse/nurse navigators (32%), and other HCPs (5%). Providers reported testing for HER2, EGFR, FGFR, NTRK, microsatellite instability, tumor mutational burden, and PD-L1 in ~55-65% of advanced GEC patients. Most providers reported HER2 testing at time of diagnosis (91%) with 18% reporting testing at time of clinical or radiologic progression and 24% when new metastases are detected. Top reported barriers to use of predictive biomarker testing were long turnaround times for tests (32%) and keeping up to-date with current testing recommendations (23%). Top reported challenges in selecting therapies for GEC were keeping up with clinical evidence (35%) and delayed, pending, or unavailable biomarkers at the time of treatment selection (20%). Clinic action plans focused on specific gaps including (1) improved HCP education on guideline-concordant biomarker use (2) improving testing turnaround times, and (3) improving referrals to clinical trials based on biomarker results. The educational intervention increased correct identification of guideline concordant treatment for stage IV esophageal cancer (48% pre- and 72% post-AF) and confidence in use of biomarkers for treatment decision-making for patients with GEC (high/very high in 46% pre- and 72% post-AF). Conclusions: While molecular testing is becoming increasingly critical for mGEC care, HCPs report challenges with testing workflows and difficulty keeping up with developments in clinical data and guidelines. Clinic-specific interventions to improve biomarker workflows and ongoing educational initiatives to keep HCPs informed of the latest evidence-based best practices in biomarkers and associated therapies can address these gaps to improve outcomes for patients with GEC.