I may be mistaken, but I think that the "avatar" you "unlock" is just that viking dude at the top of this page
whose image changes as you cross XP thresholds, not any actuall forum avatars to use.
"Whats new about this sale?
there are tons of new things and activities happening during summer
sale. you can earn XP, level up and unlock free games, as well as
awesome new versions of your summer sale avatar.
Avatar does not always mean the little image in the box beside your name. it's that character on the main screen mine changed when I leveled up. Maybe some day GOG will implement something like what you are talking about, but for now it's a static jpeg that changes when you level up.