helldweller: For anyone having this issue, exit Galaxy then open your favorite text editor (N++, Notepad, etc.) as an administrator and edit the following file: 
 %SYSTEMDRIVE%\ProgramData\GOG.com\Galaxy\config.json  
 Change the "libraryPath" value to your desired path (be sure to escape backslash characters) then save. When you reopen Galaxy your default library folder should be updated. 
 WanaGo: Thanks, this worked for me too.  
 GOG developers, you need to fix this bug!! 
 As far as I know, there isn't a bug on this.  
 Chuckster guy/girl already gave the solution.  
 Case closed.
Chuckster: I found a way that works if you are installing a new game. When it asks for the install path you can change it there and it appears to be a global setting for all future downloads.  
 For your already installed games they can just be moved and then get Galaxy to re-scan for them or manually tell it where they are. 
  That's the definitive answer to this thread.