The Gentium Basic and Gentium Book Basic fonts work fine. They are on this page: http://scripts.sil.org/cms/scripts/p...=Gentium_basic
The page you listed has the Gentium Main and Alt fonts... those don't seem to have a bold version so may be why they don't work.
Also I think we are limited to 8 fonts so make sure you don't install more than that or you won't see all the fonts.
Mine are named:
genbas.ttf genbasbd.ttf genbasbi.ttf genbasi.ttf
genbkbas.ttf genbkbasbd.ttf genbkbasbi.ttf genbkbasi.ttf