The El Capitan version of OS X brings new security mesures.

See https://support.apple.com/en-us/HT204899 for more details.

As you can see in the link above, Apple has enabled a new default security feature called System Integrity Protection (or SIP for short), also called the “rootless” feature, from OS X 10.11 onward.

The rootless feature is aimed at preventing Mac OS X to be compromised by malicious code, by allowing only code signed by Apple to modify certain files. Unfortunately, due to the way GECAMed works, we can’t sign every installation file. So GECAMed will not be able to access some importants files even if it’s not a malicious program.

As for now, you can fortunately disable this security to let GECAMed install needed files and then activate it again.

• Reboot the Mac and hold down Command + R keys simultaneously after you hear the startup chime. This will boot OS X into Recovery Mode.
• When the “OS X Utilities” screen appears, pull down the ‘Utilities’ menu at the top of the screen, and choose “Terminal”
• Type the following command into the terminal, then hit return:

csrutil disable; reboot

• You’ll see a message saying that System Integrity Protection has been disabled and the Mac needs to restart for changes to take effect. The Mac will then reboot itself automatically. Just let it boot up as normal.

You can also issue the command by itself without the automatic reboot like so:

csrutil disable

http://gecamed.lu/faq/mac/install_fails

Simply reboot the Mac again into Recovery Mode as directed above, but at the command line use the following syntax instead:

csrutil enable

Just as before, a reboot of the Mac is required for changes to take effect.