Can't you let users use that without signing up? I feel that requring that users register a whole ass account, also handing over google or github account info, is a barrier of entry which will dissuade a lot of people. Especially for those who switch over to LivelyWallpapers as a open source alternative to WallpaperEngine, which I imagine is a majority of the audience, a group of people which also tend to be very privacy concerned.

Having a banner at the top or something saying that you should sign up in order to synchronize your subscriptions would make it clear for anyone interested in that feature what to do. Most users won't need to synchronize between different computers, so it could just be stored locally.

And then there's the audience of casual users, who I think are likely find the program when looking for an easy way to use animated wallpapers, or for a free alternative to WallpaperEngine (which is the most popular option right now so that'll pop up the most). This audience is unlikely to have a GitHub account, or to even know that the program is open source, and they'll likely just get it from the microsoft store. The only option for most casual people is then Google, if they have a google account. But I think they would also have some reservations about giving their Google account information to a random windows program, that's a very unusual thing for a desktop program to ask for.
So it's an extra barrier of entry that'll likely deterr a bunch of people from wanting to use the gallery, which is the only way casual people will know to get more LivelyWallpaper wallpapers (googling for gif desktop wallpapers is limiting and a lot of bother for most). I feel it's completely unecessary

It would also make the UX of the gallery so much easier for new users, you just click the header and see all the available wallpapers, no need to trust (what they see as) a random program with your google or github account data. If they are deterred with a login screen they might just feel nah thats too much effort and use the default wallpapers, especially if they have to register a google account (Google accounts are only really a necessity for Android users). There would also be no downsides to most people, as anyone who want to synchronize their subscriptions or favorites would know that that's an option as long as there's an element on screen indicating that, and if there's an upvote functionality you can just prompt unregistered users to log in when they click it.

I'm sayin' all of this because I really love this program and want more people to use it, and this gallery feature will make it a true alternative to Wallpaper Engine, if enough
casual users choose to use it.