fix(gtm): expose gtag globally (#524)

Co-authored-by: Dan Larrea <d.larrea@gitlab.com>

Author: DanLDU

src/runtime/registry/google-tag-manager.ts

@@ -167,6 +167,9 @@ export function useScriptGoogleTagManager<T extends GoogleTagManagerApi>(
                 (window as any)[dataLayerName].push(arguments)
               }
 
+              // Assign gtag to window for global access
+              (window as any).gtag = gtag
+
               // Allow custom initialization
               options?.onBeforeGtmStart?.(gtag);