Google says Gemini will replace Google Assistant on Android phones in 2025, except on phones with Android 9 or earlier and that don't have at least 2GB of RAM.
Gemini is hardly an uprade to Assistant, though. Gemini can't do any task automation (aka the one thing Google Assistant could do) because it's a LLM that most likely doesn't even 'know' that it's being run on a phone....
I don't know if they can really do that since Gemini is just an LLM at its core. Maybe they could integrate Assistant functionality into Gemini on Android that's then triggered by certain keywords, but at that point why not just keep both Assistant and Gemini?