Nowadays every programmer uses git as the preferred version control system or at least is familiar with it. Some use the command line interface, others favor visual git clients. My favorite is the powerful Tower. It is easy to use, informations are quicker to find and it offers most of the features you will need to work with git.
But the app has one major drawback. If you have a development workflow that utilizes git hooks to automate certain things, it could be that Tower shows you some errors.
If one of the git hooks is using ruby and requires some special gems, Tower does not know them, if you have them installed in your custom ruby environment. Mostly ruby is managed with either rvm (ruby version manager) or rbenv.
Tower ignores these versions and makes use of the system default ruby version, which is installed with your Mac OSX. On OSX Mountain Lion this ruby version is located here:
Install all the gems you need for this ruby version and Tower will work as expected.