add support for Gemini 3.1 Pro

[vunguyen-207]

Feature Type

Enhancement to existing feature

Description

I could not access the Gemini 3.1 Pro model through the Proxy with antigravity/gemini-3.1-pro-preview

Use Case

Make the proxy more customizable and adaptive

Additional Context

No response

Pre-submission Checklist

• I have searched existing issues to ensure this is not a duplicate

[Mirrowel]

This means you were saved from getting banned. Antigravity cracks down on multiaccount and proxy use, so until some other way matures you really don't want to use antigravity models.

Until then there is no point in doing any work on antigravity features.