GOG, Origin, Uplay...

Not really regularly in any sense. Mostly when the game isn't available in Steam or if the price difference is really worth it. Most of my games are on Steam and I think that there won't be change on that in the future, unless Valve does something really outrageous or my account gets removed (is it even possible in any case?) etc.

I don't even buy games in a regular basis. But, if I compare the games I have bought in GoG to the ones I bought on Steam, the answer would probably be no. Even so, I have some sweet games in my GOG library, like The Witcher 3.

I buy and play games on GOG a lot. I personally favor it over Steam because of the customer treatment and no DRM.

I have a uplay account but that's it other than Steam. The only games I have on uplay are the free ones they give out every months. I don't really see a reason to use gog if you have Steam.