It is possible that one of the fonts that you have installed is interfering with the fonts installed by the system. On an Apple Macintosh, you can go into FontBook and do {{man menu|File -> Restore Standard Fonts... }} This will remove any non-default fonts to a (Removed) folder (so you should not lose any of your fonts) and you should then make sure you have quit Gramps, and then try Gramps again (you may need to shutdown and restart, but this was not necessary when I did this).