It’s a package manager. It handles downloading files and updates, installation and patching, and verification.
It integrates various GOG services, like cloud storage for save files.
It can set environment variables and pass arguments to launched games.
Besides, a Linux-native port doesn’t need to package anything. It can simply mark Wine/Proton and various compatibility solutions as dependencies. Lutris, for example, is still a great utility even if it doesn’t use the packaged Wine versions: all it really needs to do is execute some program in the correct runtime environment with the correct arguments.
Even if Galaxy is running under Wine:
Besides, a Linux-native port doesn’t need to package anything. It can simply mark Wine/Proton and various compatibility solutions as dependencies. Lutris, for example, is still a great utility even if it doesn’t use the packaged Wine versions: all it really needs to do is execute some program in the correct runtime environment with the correct arguments.
I think reinventing the wheel is pointless. Just indicate that Heroic Games Launcher fully supports GOG, put that on GOG’s website, job done.