It's available to Pro in the Gemini app (and the gemini.google.com). It isn't available in antigravity. They know that the people who are using these tools for coding are most likely to cough up for the Ultra subscription.

EDIT: Okay, the AI plans page claims under pro - "Expanded access to Gemini 4 Argon in Google Antigravity." So maybe it is just a matter of it rolling out. Though Google has a long history of serious confusing and misinformation on those tier descriptions.

EDIT2: And they changed it again, and now it is explicitly excluded from antigravity for Pro users.