[PSM Interop] Add GKE_CLUSTER_PSM_GAMMA option to the list of clusters (#34150)

Reminder: changes to this file are picked up immediately by all
tests/all branches/all langs.

tools/internal_ci/linux/grpc_xds_k8s_install_test_driver.sh

@@ -50,6 +50,10 @@ activate_gke_cluster() {
       GKE_CLUSTER_NAME="psm-interop-security"
       GKE_CLUSTER_ZONE="us-central1-a"
       ;;
+    GKE_CLUSTER_PSM_GAMMA)
+      GKE_CLUSTER_NAME="psm-interop-gamma"
+      GKE_CLUSTER_ZONE="us-central1-a"
+      ;;
     GKE_CLUSTER_PSM_BASIC)
       GKE_CLUSTER_NAME="interop-test-psm-basic"
       GKE_CLUSTER_ZONE="us-central1-c"