i just did a new configuration of gemini flash 3.6 and all i had to add to do was set the api key to use (which in my case is called 2026-07-02). i didn’t change anything else from the default setup for flash 3.6 and it tested successfully and worked fine:
but the api key credential is using the gemini api service, not vertex.
my GCP configuration looks exactly like what you linked in the disco-docs screenshots (those are screenshots of my GCP config!).