It looks like Pro can use argon in the Gemini app, but not in antigravity:
https://one.google.com/about/google-ai-plans/?icid=microsite...
https://one.google.com/about/google-ai-plans/?icid=microsite...
It seems to be rolling out gradually.. not available at once to everyone.
It's crazy to me how care there is in most of these vibe-coded vibe-coding harnesses. I don't like oai's models much but I have to say their UI has by far the best attention to detail of any major player.