The window just keeps me an error message. Can't enter my steam account :/

Using Mozilla and Chrome didn't work.

Literally this forum is flooded with the same topic...

Long story short, Steam's API limitation prevents so many requests over the span of time.

You can only wait.

Reference: https://www.gog.com/forum/general/why_you_cant_connect_to_get_your_games_from_steam

What's the error message? If it's something like "Due to exceedingly high demand this may take up to several days, please check back later. Thank you for your patience" then it's not an error - it's just telling you that you're on the queue to have your Steam account scanned for eligible games. You can safely close the tab, but make sure that you keep your Steam account public for now.

I have been stuck at;

"YOU ARE QUEUED FOR GOG CONNECT

Your accounts will be connected as soon as possible. Please check back later."

for a couple months now.

When I first used it, it worked fine but now nothing. I wanted to add my Witcher games to my GOG Library as they are sold on GOG.

Wrong feature for that:
https://www.gog.com/witcher/backup

Thank you for that, I've added Witcher 2 using this so it was very much appreciated. However, I am unable to add Witcher 3 & its DLC this way. Steam never provided a key & it lacks the option to view a key for this game, unlike other games in my library.

There is no way to redeem W3 here, only 1 and 2.

As of today Witcher 3 from Steam can't be redeemed here on GOG. May be possible sometimes (far?) in the future but currently it isn't. Only is supported for TW1 and TW2 atm.