This might be a dumb question, but having used no other platform other than Steam, I really have no idea how platforms relate to one another - or if they do to begin with in terms of how games are purchased / activated.

So if I buy something from GOG, can i activate it on steam instead?

Specifically, I want to buy a dlc from GOG that belongs to a base game that i have on steam. will that work?

No. GOG games are DRM Free and as such do not require a client to play and GOG does not give Steam keys. GOG however does have a optional Steam like client called GOG Galaxy. Generally speaking... no.

No. DLC purchased on one service won't be compatible with a base game purchased from another. GOG won't even let you go through with a purchase of a DLC unless either your account contains the relevant base game, or you mark the impending purchase as a gift order. (And you likewise couldn't redeem that DLC gift code without owning that game here.)

Unlike a lot of other stores, GOG distributes everything they sell (meaning no third-party codes/"keys"). The only way to get a Steam key for a GOG game would be to contact the developer or publisher directly and ask them for one (providing them with some proof of purchase, of course).

GOG does however allow you to get DRM free copies of games you've already bought on Steam. Use GOG Connect at:

https://www.gog.com/connect

to see what is currently being offered. It's a great feature, and is updated every so often.