I have connected some accounts to Galaxy 2.0, every single of them uses its platform's own login system but Steam connection somehow uses a different login window instead of Steam OpenID. I don't wanna enter my account credentials to a 3rd party login window and I just wanted to ask why only Steam is different.

Came to the forum looking for an answer to this, hope someone sees it

To add a bit of information, there is an open issue in github about this, it has some time but has some comments from the developer/manteiner? of the plugin:

https://github.com/FriendsOfGalaxy/galaxy-integration-steam/issues/2

also, just to make it clear because me for example missed it: that steam integration is not developed by gog but by some dude on github.

Just to make it even clearer - non of them are except Xbox

Yeah, only MS integration is official, all the others are user made. However I'd still use Steam integration if it used regular Steam auth API (OpenID) like on GOG Connect https://www.gog.com/connect