It looks like the key idea here is that a GitHub repo can have a gemini-extension.json file which specifies some MCP servers and a GEMINI.md file, then you can tell Gemini CLI to add that repo and it will fetch and configure those details.

Here's a commit I found adding one of these for the Google Maps platform: https://github.com/googlemaps/platform-ai/commit/95c4efdb43c...