diff --git a/packages/cli/src/config/auth.test.ts b/packages/cli/src/config/auth.test.ts
index 2360cf60e76..8daf5184efd 100644
--- a/packages/cli/src/config/auth.test.ts
+++ b/packages/cli/src/config/auth.test.ts
@@ -30,6 +30,7 @@ describe('validateAuthMethod', () => {
     vi.stubEnv('GOOGLE_CLOUD_PROJECT', undefined);
     vi.stubEnv('GOOGLE_CLOUD_LOCATION', undefined);
     vi.stubEnv('GOOGLE_API_KEY', undefined);
+    vi.stubEnv('GOOGLE_GEMINI_BASE_URL', '');
   });
 
   afterEach(() => {
@@ -92,6 +93,22 @@ describe('validateAuthMethod', () => {
         '• GOOGLE_API_KEY environment variable (if using express mode).\n' +
         'Update your environment and try again (no reload needed if using .env)!',
     },
+    {
+      description:
+        'should return null for GATEWAY if GOOGLE_GEMINI_BASE_URL is set',
+      authType: AuthType.GATEWAY,
+      envs: {
+        GOOGLE_GEMINI_BASE_URL: 'http://127.0.0.1:8080',
+      },
+      expected: null,
+    },
+    {
+      description:
+        'should return an error message for GATEWAY if GOOGLE_GEMINI_BASE_URL is not set',
+      authType: AuthType.GATEWAY,
+      envs: {},
+      expected: 'When using Gateway auth, GOOGLE_GEMINI_BASE_URL must be set.',
+    },
     {
       description: 'should return an error message for an invalid auth method',
       // eslint-disable-next-line @typescript-eslint/no-explicit-any
diff --git a/packages/cli/src/config/auth.ts b/packages/cli/src/config/auth.ts
index 1ca07f98eb4..6fd7d5b0d2a 100644
--- a/packages/cli/src/config/auth.ts
+++ b/packages/cli/src/config/auth.ts
@@ -45,5 +45,13 @@ export async function validateAuthMethod(
     return null;
   }
 
+  if (authMethod === AuthType.GATEWAY) {
+    if (!process.env['GOOGLE_GEMINI_BASE_URL']) {
+      return 'When using Gateway auth, GOOGLE_GEMINI_BASE_URL must be set.';
+    }
+
+    return null;
+  }
+
   return 'Invalid auth method selected.';
 }