I know I've found Gentium Basic in all 4 required styles, if that's the same font you're referring to.
http://scripts.sil.org/cms/scripts/p...=gentium_basic
If there's no really no bold, italic or bolditalic for the font, I would find a similar font you could live with and just rename the files to Gentium-Bold.ttf, etc. so the reader sees all 4 styles present. Maybe sub in the Gentium Basic fonts and just rename them all so they match as one font family.
It's been awhile since I tried using only the -Regular font, but I'm pretty sure the reader ignored the font and used the default in that case.