Gastric or Gastroesophageal Junction Adenocarcinoma (GEC)

The purpose of this protocol is to give continued access to tucatinib eligible participants. It also enables ongoing safety follow-up for those who continue to benefit from the study treatment. These participants were part of Pfizer-sponsored tucatinib parent studies that will be closed. Additional follow-up safety information will be collected. This will allow further understanding of the safety profile of tucatinib in participants who continue to receive the study treatment.