The Gentium font, and other fonts on that web site, are open source codes. If you like to program, the folks at SIL will let you help upgrade their fonts. We do not have any experience with the Gentium font, or the other fonts on the software.SIL web site. We have seen good recommendations for the fonts from this website. These fonts are more Latin, Greek and Cyrillic based, and are designed to be use on Mac, Windows, and Open Office/Libre. If you are looking for a font to type Latin, Greek, and Cyrillic based languages using an English keyboard, this may be the web site for you.
Here are the links if you are interested...
Home page for the software.SIL is: software.sil.org/
For the Gentium Greek font: software.sil.org/gentium/
PRACTICE WRITING GREEK LETTERS