Can we take two steps back and address a simple question - what is it you are trying to do.
- If you just want to run gramps, install from the dmg file. Nothing else needed.
- If you want to write an addon, you do not need a development environment other than what is installed by default on Mac.
- If you want to build from source, you need to install a dev environment. Best description for this is Mac OS X:Build from source:Application package - Gramps
- If you want to create your own module repo for your custom addons, you will need a dev environment.
Assuming you want to write an addon, you can create it in the default directory for addons and do edit/test cycles pretty fast. You can review the SampleReport addon Sample Report for new developers as an example.