In my chef-repo I did:
knife cookbook site install ghmac knife cookbook upload ghmac #... add 'recipe[ghmac]' to my workstation role ... knife ssh role:workstation 'source .rvm/scripts/rvm && chef-client'
Since my Mac OS X workstations all have role[workstation], they all got GitHub for Mac installed automatically. I did have to do the configuration steps through the UI, and that made me cry a little, but I got over it. The application itself is quite nice.