Possible answer to the question I asked:
http://images-4.gog.com/036fa36e4f1cabc9ee87beb200d8e0295ddb4bb9576c39f98980d149bad38f74.png?_ga=2.205011481.84326263.1537686271-1518449373.1507615682

Maybe GOG was designed to have a purple background, but only showed those two tiny bands on the page.

No pop no style, I`m strictly roots!

If GOG didn't want useless things around I'd have been perm banned years ago.

It represents GOG Galaxy (it is GOG's colour now) and is there for aesthetic purposes (and subliminal message maybe).

I'm betting it's a leftover "sliver" from some image that used to lie below/behind the banner. In other words, it's the very top of another image... perhaps a background image... that GOG used for something once... and forgot to remove it. It's not uncommon to "slice" up an image for a website, so those slices will stick out, or not be obstructed, or just to help construct the "look" they're going for.

To misappropriate Dickens, "[It] may be an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of underdone potato."

wow, people really do complain about anything and everything.

Why criticize the one part of the current design that isn't absolute garbage? It's not useful, but at least it isn't making the store a fucking chore to use.