Just wondered if it is or not?

Yes, it is!

Just keep in mind, if you got the key from somebody else, it would be polite to ask if he's ok with it ;-)

Some might cry abuse, but I see nothing wrong with it. Plus, we do it openly and no one complained. So my guess has always been it's allowed.

While there is no trading system implemented you can trade GOG codes by gifting each other the codes.

Yes, it is

so its not against t and cs?

Of course, you can only trade unused codes. >.>

Deep down, they burn with hatred.

But they let us do it anyway. Aren't they just the most awesome latent homicidal maniacs?!

You might even trade partially used codes, which is actually pretty awesome. Well, for giveaways at least. ;-)

as others have pointed out, yes GOG allows trading but please do so carefully as there have been issue with Scammers so try to keep trades to users with a high rep and were here before Oct. 2012 if you do trade with newer members check them out first, make sure their name does not have anything weird attached to it such as a .(Thespian* is a exception to that) and has low rep points(usually around 1-6) and has a recent sign up date. One particular scammer would take the name and forum avatar of well known users with decent rep and add a period at the end in a attempt to use their name and reputation among the users to scam them, these usually apply primarily to games on Steam but you never know.