You don't have to. You can auth via web browser or you can use API. I made an app with web auth and with API. The web auth used the Flash model, switching from Pro, stating that auth was too slow for responses (meaning busy). I used API with Pro in another test. Both apps looked and functioned the same. My prompt with both was "build a Twitter like app". It built an extremely basic version. Dork and Claude build a much more advanced version.

I feel that Google's Gemini CLI is good for demoing, testing or tinkering around. To get started for free. Then if you want to upgrade and spend money with other models, you can.