In theory, you should just see the same apps in Discover and in GNOME Software, assuming they both can access/install native Alpine and flathub apps. (And I recall they did at some point in time, but breakage unfortunately happens, and I mostly use the terminal to install and remove things...)
With Ubuntu Touch apps it's different, the Open Store is its own thing. I want to look into whether adapting this to postmarketOS when I can find the time.
Regarding few apps: Yes, it could be more, yes, there are gaps, but we're doing somewhat okay: https://linuxphoneapps.org/