nciceman: I had a lot of weirdness with this, so let me list ALL the steps that finally got it working for me:
1. Completely close GOG Galaxy 2.0 if it is running. This means make sure it is not in the taskbar either. Do this by clicking on the gear icon in the upper left of the GOG galaxy windows and selecting exit.
2. Go in to your %LOCALAPPDATA%\GOG.com\Galaxy\plugins\installed\ folder and remove any existing Rockstar plugin.
3. Start then fully exit the GOG Galaxy 2.0 client again. This insures the client has forgotten all about Rockstar.
4. Restart GOG Galaxy 2.0. Click the gear icon in the upper left and go in to settings. Confirm there is no Rockstar plugin entry.
5. Go into the Search GitHub field and type 'Rockstar'. Install the plug-in.
6. Once installed, attempt to connect. I checked the Accept Cookies box to be safe. Go through the login process. This will fail at a black screen; this is expected. Close the login window then completely exit GOG Galaxy again.
7. Go to your %LOCALAPPDATA%\GOG.com\Galaxy\plugins\installed\ folder and then go in the Rockstar plugin folder and edit the config.py file. Find the line that begins with "start_uri" (that's with an i, not an l). At the end of that line, inside the ending quotes, add the text "&returnUrl=/" (this time it is url and not uri). Save your changes.
8. Restart GOG Galaxy and attempt to connect. This time, after a brief pause with a black window, it should work.
Hi there. I wanted to say thank you for writing these steps. This fixed the login problem for me that I was having for months, which caused me to give up on having the Rockstar launcher integrated with GOG.