The aim of Comet is to eventually get it to work with projects like the Heroic Games Launcher and Lutris, to get their GOG support into even better shape with Wine / Proton on desktop Linux and Steam Deck. So in a way you could compare it to Legendary for Epic Games, giving a minimal and platform agnostic SDK.
A developer reached out to note they're now after more testers to play around with it, specifically on desktop Linux with Steam Deck testing planned later on.
So Comet will provide support for the likes of achievements, stats, friends, leaderboards and more. It's not finished yet of course, with some features under development and some not there at all yet but it's impressive work and continues to be an example on how the FOSS community are showing certain companies how it's done with a little community effort.
See more on the GitHub.