PaladinFenris: This is for everyone, who accidentially checked the box "Don't show me this message again" when asked for confirmation of opening the external application GOG Downloader, when you try to add a game to the Downloader in Opera.
I had the same problem, that checking of the box for not showing me this message again gave me some hard minutes with my Opera and GOG-Downloader.
And I got a rather complex, yet simple solution.
Open your "operaprefs.ini" (while Opera is closed) and search for the line(s) with the exact text "gogdownloader" (without the " of course) in it. Delete that exact line(s), save the file and open Opera again... and you should be able to add games to the GOG-Downloader again. But never check that box again. It's either that message box, or you have to download manually (or via another browser).
That additional click was tiresome to me... after searching for about 30 minutes aroud the internet, it now became a very friendly thought, the additional clicking for that download :-)
Hope, I could help some of you!
I need this solution, but for Chrome. Anyone able to help?