Hm. If you haven't changed the conf file, you can try starting the client with a new, temporary, empty wallet. Save the original wallet in some other data dir, start up the client, if it starts, shut it down and put the original wallet back again. Or, you can try manually adding txindex=1 in the conf file. That overrides gui checkboxes.
Tried both of those and no dice. That pref setting must come from somewhere else on the Mac. Tried deleting ~/Library/prefrences/...guncoin... and didnt fix the issue.