You're going to need to be more specific. You can use installed GE versions of proton in game mode without any plugins at all.
What, exactly, are you trying to do?
If you mean downloading, there is wine cellar
Like the others said, you should probably clarify your question, it's kind of unclear what you're asking.
If you mean using GE Proton versions, they are just available in your game management options in Gaming mode once you've installed them, just like any other Proton version is.
If you mean downloading / installing them, I'm not sure what your objection would be regarding doing that in Desktop mode... using ProtonUp-Qt to do it is really quick and simple.
If it's the length of time for booting back and forth between Gaming mode and native Desktop mode that's your pain point, you can always just use Nested Desktop to flip over to your Desktop mode functions while you're in Gaming mode and eliminate most of that wait time. I cover how to set that up in this post here.
