As previously announced for the Pixel 8, the 8a will be getting on-device Gemini Nano — in a future update — as a developer option that users can enable for Recorder Summarization, better Gboard replies, and Magic Compose in Messages.
I'm guessing there is a negative in enabling this feature if it's going to be tucked in developer settings. Likely increased app reloads and the phone running hotter.
Android sandboxes apps anyway, what GrapheneOS does is allow you to install Google Play Services in a sandbox so you can use it without granting it the admin-level permissions it usually demands.