OS X 10.9.5+ - "can't be opened because the identity of the developer cannot be confirmed" error message
To get around this issue, please right-click (or Ctrl-click) the game and select "Open". This should only be required once, and in the future the game should launch normally

Game "is damaged and can't be opened. You should move it to the Trash."
a) Redownload the game using the newest version of the GOG Downloader:
https://secure.gog.com/downloader
To download, make sure the "GOG.com Downloader mode" is switched to ON before clicking the "Game installer" button. After the download is complete, reinstall the game.
b) Use this workaround if using the Download didn't help:
- Go to Applications -> Terminal
- Type in:
xattr -d com.apple.quarantine
but don't press Enter.
- Drag the application to the Terminal window, this should add the path.
- Press Enter.

Game "can't be opened because it was not downloaded from the Mac App Store" error message
a) System Preferences -> Security & Privacy -> unlock to make changes -> select Mac App Store and identified developers.
or
b) Right-click (or cmd-click) the game -> Open -> confirm Open.

Right mouse button not working in-game
Your mouse's right button may be set to Primary Click by default. To change this, go to System Preferences -> Mouse.
This window's appearance will vary depending on your OS X version and mouse model, but you should set the mouse's right button to "Secondary Click" or "Secondary Button".

Access violation errors, or problems with saving / loading games.
Go to Applications -> Utilities -> Disk Utility -> Repair Disk Permissions

Apple cursor stays on top (does not disappear) after clicking the game's window
Disable all third-party user input applications, like Better Touch Tool and similar.

Q: Where can I find the End-User License Agreement (EULA)?
To read the EULA, right-click (or cmd-click) the game's .APP file and click "Show package contents". The EULA can be found inside.