Sooo, I have Fallout Collection (1+2+Tactics) boxed version and I would like to exchange this GOG version of Fallout 1 (I don't need) if someone's interested.
You're free to offer whatever you like as long as the game is in the GOG catalog.

Not sure quite what you mean, Do you want to trade an unused GOG code because you have the boxed copy?

If the game is on your shelf under my games and not a gift code I dont think it can be traded

Edit: if its a gift code you may want to search out the topic called classifieds were trading happens

You should definitely explain a bit what exactly do you have to trade.

Yes I want to trade GOG version from my games (not a gift code).
If that can't be traded right now, I hope GOG will make it possible in near future.

You can't and they won't since GOG deals with DRM free there is no way they can guarantee someone wont exploit the system by keeping local copies after the game has been removed from their accounts.

You're probably right, I haven't thought about it in that way.