Yours isn't a consumer rights issue. You can't dictate how they run their business and how their shop software has to work. That's unfortunately a case where you can voice a complaint, make a suggestion to improve it and hope they will hear you out and implement the change(s). That's what online stores with a pro-consumer approach usually tend to do. GOG could make changes to their software if only they wanted to instead of a standstill because: The original programmers have long since left the company ... Sad excuse, no?

I'm not the person to tell you what and what not you should do. I suggest you don't complain to those being able to change the least, those being in the middle having to take all the complaints and harshness: support staff. They can't change a thing and for real change the filthy rich owners would have to invest money to modernize this mess. Kiciński, Iwiński, their board members, instead of sticking up things their shareholders behinds, they should put money into development to improve the convenience of using this store.

Your general option in this case is binary: keep complaining, boycott GOG or move on to find a greener pastures.

And as I said earlier, I'm not going to pay for something I already own just because of the store's technical flaws.

I can only make an exception if they give me a 99% discount coupon. (: