I purchase my games exclusively on GOG but do buy games for friends on Steam if they insist that they want to have their games there. I always offer to buy them from here first, of course. I am a tad conflicted about it all, however.
On one hand, GOG is free of filthy DRM and is all about options. Galaxy, for example, is optional, and the features within are optional as well. Being able to turn off achievements and game time tracking is a blessing for my soul. I can enjoy gaming again! You see, ignoring achievements and game time tracking is not an option. Turning them off entirely is the right way to go. Out of sight, out of mind. Steam just does not let one do that and that irks me greatly.
On the other hand, Steam is actually available on Linux, unlike Galaxy, and Valve has the vast majority of its games on Linux, unlike CD Projekt Red. On top of that, Valve is really pushing the development of the Linux ecosystem, hiring developers for the open source graphics drivers, encouraging the use of the Vulkan API, and more. I may not buy games from Steam for myself, but I do have a Steam Controller and will gift games via Steam if friends request it. Apart from Galaxy missing on Linux, you often find games missing their Linux clients on GOG as well, despite being readily available on Steam.
In conclusion, both platforms have their advantages. If GOG could get its Linux support sorted, it would be the perfect platform. Until then, I will support Steam in other ways that do not involve buying games for myself.