How much do I gain from switching to it instead of say, remaining on the Stock Android?
Edit: This looks comprehensive — https://staging.grapheneos.org/features
Beyond that, most of the other advantages will be less visible. They have hardened memory allocators that make various classes of security beaches significantly more difficult. There's a lot less superfluous background services eating resources. All that and more are listed on their website. It's well worth a read.
Google Wallet does not work, therefore I cannot use my phone to pay wirelessly with my Neon card, which is a shame.
The only apps I had trouble with were Twint (had to install it with F-Droid, as Play kept telling me it was not compatible with my device), and... the McDonald's app (which forces me to move my fat ass to one of their kiosks to order my food instead of doing it from the table).