Why don't you contact GOG support? Some other games have already been patched for El Capitan.
How do you do that, I have tried to find a way but they make it nearly impossible to do so, guess they don't want people complaining all the time.
First read and follow these troubleshooting steps
and if that doesn't help send a support request ticket
(click on the previous linked text) clearly describing your problem. If you don't receive a response in a few days (don't count weekends as they are not working days), don't open new ticket but always reply to the existing one that was automatically sent to your e-mail upon submission.