I've been buying the few games I play on GOG, so that means I can't sell my CS:GO items anymore.

I ran into this after not buying anything new in a while (really don't need new games) If you have anything in your steam wallet from past items being sold, you can use that for the requirement of buying something, look for just a cheap classic, usually they can be $0.99 or so on steam sales. I found that works to get back to being able to trade again