As stated in Google AI emails, they recommend AI project that only want to use free tier to disable the Google Cloud billing. So I did create a new project with no billing account.
Vertex AI does propose a free of charge offer: Gemini Experimental. You can query a new model from Gemini 1.5 family for free with a rate limit of 2 queries per minute. I wanted to ensure I won't pay anything so I tried Gemini Experimental in my billing disabled project. What an unpleasant surprise to see that Gemini Experimental requires Vertex AI API, and Vertex AI API requires billing.
It feels like I can't protect myself from unintentionally billing my account when testing Gemini Experimental.