From the source
Google integrated Antigravity, its agent-first development platform, into the Gemini app for macOS as part of the Gemini 4 Argon rollout.
The integration adds a Liquid Glass pill in the top-right corner that launches the full Antigravity experience.
Gemini 4 Argon requires AI Pro and above in the Gemini app, while Antigravity access requires AI Ultra.