According tho its website, Gentium is a typeface family designed to enable the diverse ethnic groups around the world who use the Latin and Greek scripts to produce readable, high-quality publications.
The license is quite liberal. It avoids the “viral” effect of GPL: embedding the Gentium font in the PDF you redistribute on a numerical form won’t turn on the GPL for the PDF.
Bad news: the OFL license doesn’t seem compatible with the GPL v3 (documentation is unclear). So you can use Gentium fonts but you’ll have to download it with your bare hands, as it cannot fit in a Novelang distribution, which is released under GPL v3.
Good news: the beautiful Junicode font will become part of Novelang. Junicode (short for Junius-Unicode) is a Unicode font for medievalists. It embeds various amazing characters like old English and runes. But as a great classical serif font, it’s a nice replacement for boring Times New Roman.
But the Junicode license, which is GPL v2, has no exception about embedding so if you embed it in a PDF (what Novelang always does) your PDF will be redistributable under the GPL license.
Better news: the Linux Libertine font, bundled in Novelang for a long time, is dual-licensed, under both GPL and OFL, so you can use at no legal risk.