Here is perhaps a slightly more clear set of installation instructions for GearBox 7.2 on Mac OS X El Capitan.
1. After you download the .dmg file, double-click on it. If you go through the steps, eventually, you will see that your hard drive is not usable for this installation,
as several people noted. Do not despair.
2. On your desktop you will see a GearBox disc, right-click on it and open in Finder.
3. From this point on the instructions that were posted previously (by a Polish user) are easy to follow. Right-click on the package and click on Show Package Contents, then go to the GearBox file and install just that one. Then the program will execute the installation of the GearBox application.
Note, that this just installs the application. Then you can try to get your hardware to work with the application. Other people have posted here on those other issues, as they encountered them. Good luck!