Gentium Book Plus is the new version of the reduced character set families, Gentium Book Basic. This 'Basic' familie only cover a limited range of Latin, and none of Gentium Book Plus's Ext Latin, Cyrillic, or Greek (both modern and ancient).
Gentium Book Plus now extends the same 4 styles/weights of the previous Gentium Book Basic and to the full character set that complete Gentium Plus (also on github).
This update brings many other improvements, including a very significant improvement in hinting for Windows users and WOFF2 versions. These fonts cover a far greater character set than the Gentium Book Basic, so are larger, but since GF subsets the difference won't affect users.
You can find here the Gentium Plus, a very similar family that has a slightly lighter weight.
To contribute, see github.com/silnrsi/font-gentium.