Runs on OS X is a bit of an exaggeration though. It can supposedly be installed through MacPorts, but the two times I ever tried, MacPorts installed about half of normal Linux distro, before failing on some broken dependency.
If you want to go through several hours of compiling and several hundred megabytes of dependencies, with your fingers crossed that things won’t inexplicably break 3 hours in, feel free to try to install Gobby on a Mac. Personally, I’ll stick to SubEthaEdit, or whatever web-based editor, if I need collaboration.
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I really think desktop collaborative editors should try to provide a web-based alternative front-end, for just this reason, that it’s nice for everyone to be able to contribute, and there’s no better way to ensure that than through the web.