Mixed bag, with its pros and cons
Definite pros, no DRM, period. It has its platform if you wanna manage your games, but it's completely optional. Good selection of retro games, modified to run on modern rigs, fair selection of indie stuff as well, not so much for AAA stuff.
Cons, no workshop style functionality for mods. Less active community, due to smaller customer size, but overall friendly anyway, as far as I've experienced.
The quantity and level of sales are about even between the two.
Personally, I try to get stuff on GoG if I can, and go with Steam if the game has a heavy modding community.
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