Purchasing on Steam does not mean your game comes with DRM. Many GoG titles are DRM-free on Steam:
https://www.pcgamingwiki.com/wiki/The_big_list_of_DRM-free_games_on_Steam
Viewing post in itch app Steam shortcuts and headless launching comments
Thanks for the reply and the clarification and sorry for the misunderstanding because what I meant was that I want to support companies who advocate DRM free gaming (like GOG, itch.io and some other small companies) and therefore I only buy my games there and never on Steam. Furthermore "without DRM" means that I still need the Steam client to install the game and that's what I don't like and I also don't like to pirate a game to get a standalone installer because I want to pay the people who made the game to get the installer officially.
Yeah I respect that dedication, I tend to buy on GoG and itch but as a Linux gamer I sadly find Steam much more conveniant, less tinkering to get my games running than using Heroic or Lutris. I do support DRM free and that's why I sometimes buy games on GoG that I already "own" on Steam even if I don't plan on playing them in the near future. However the sad reality is that most publishers treat GoG users as second class citizens and many updates, languages or DLC never make their way to GoG:
https://docs.google.com/spreadsheets/d/14haGnU2kcY6wV6ssBeyW6_mMk6kkVqZX-QvMKlgX...