I just installed gedit and its spell checker on OSX. However, whenever I come to "Set Language", to set it to English so I can do spell checking, I get an empty list, and English does not appear there. In general, the spell checker does not work right now because it has no dictionary.
I added the latest aspell package and enabled some english dictionaries in the spelling control panel (settings). Still no dice. The mac knows how to check spelling in SimpleText but not gedit. No clue in the spell checker documentation for gedit plugin. That seems like a rather serious omission! :^)
The Set Language menu item does come and go with the plugin.