Sorry, we get a lot of drive-by and troll topics like this. There's so much wrong with your OP.

There is no legit way to buy GOG keys for most games. You either buy from GOG or get a key as part of a promotion (there are exceptions. Cyberpunk 2077 isn't one).

You don't need any client to download games from GOG. They have one, and it is useful, usually. In no way will you ever need to use Steam to redeem or activate a GOG game.

The install error can be managed, but something very weird is going on with your purchase. Your anger seems to be misplaced. You gambled on a 3rd party gray market site and got burned. Don't be mad at GOG for your badly researched shopping habits. Sorry you rolled the dice and lost, really I am.

CP2077 is one of those rare games available as GOG keys from Humble Bundle.

Thanks for the correction. Though it doesn't sound like he got it from Humble.

True enough. Not that it matters a great deal... yet. What matters at the moment is not where the code came from but rather that while it was successfully redeemed (profile seems to indicate so) the owner has had issues getting the game installed.

Which in this day and age shouldn't be such a problem. Such a thing should be quick, easy and painless. Galaxy is definitely not as mature/robust/polished/efficient/optimised as it should be this far into development, and yet it desires to be "more" than a simple but effective game client for its customers. Like much of GOG's infrastructure, its fundamentals - its basic levels of service and utility - aren't up to scratch, yet too much development time is being devoted to extras it can scarcely afford to provide.

In other words, GOG: stop concentrating on dessert when the main course isn't good enough.

Well, that the first thing he did was try to link Steam to Galaxy as a requirement to install his game is fairly telling that this is almost 100% assuredly a user error.

https://i.imgur.com/vl06xzU.gif