Can you add games to your steam library

No, GOG is a Steam competitor. There may be a development studio or two that allows you to do that. But as a rule the answer to your question is no.

But can't you add them as "non-steam games" to your library?

You can add a shortcut to your library. If you wish.

GOG is an independent retailer, not a Steam key reseller.

GOG.com doesn't give Steam keys (as Kristian said, GOG.com is the competitor of Steam, and opposite to any kind of DRM), but just in rare and very few occasions, if you bought a game from here and contact the developers with a proof of purchase, they can send you a Steam key. But GOG.com will never deliver Steam keys.

Although, you can still add any game to the Steam client through the "Add a Non-Steam game to My Library..." option.

I love how every time there's a big sale/promo on there's that one guy... *le sigh*

That one guy who dares to wonder whether or not he can redeem his games bought here on his favourite distribution platform? Yes, how can he, the bastard!

That's a bit of a stretch when we're talking new members.

That and any site selling Steam keys tends to specifically mention Steam in its requirement and/or on the store page.

also a very nice way of welcoming a new member... not that we would like to be seen as inclusive and friendly.

In the same way you can add Origin games to your Steam library.

Also INB4 Licurg comes in and says "FUCK STEAM!!!"

Well, I never claimed to be Mr Nice Guy either. =P That said, a little research can go a long way. I guess I'm also a bit tired of people coming here expecting that everything should revolve around Steam. I use that service, I don't mind that service, but a lot of people overpraise it to hell and back. On that note, while I prefer GOG it certainly has its flaws, too, and those among us who do praise it above all others tend to irritate me just as much. ... I just tend to keep my mouth shut about such things most of the time. *shrug* Oh fucking well.

I thought his usual greeting to new members involved Sacrifice and the importance of buying it.

Congratulations, you are king of le memes

Fuck DRM-riddled $t€am.

And have you bought Settlers 2 Gold/10th anniversary?

You won't be unhappy if you do ^_^