Gentium Plus is the new version of the two reduced character set families, Gentium Basic and Gentium Book Basic. Those 'Basic' families only cover a limited range of Latin, and none of Gentium Plus's Ext Latin, Cyrillic, or Greek (both modern and ancient).
Gentium Plus now extends the same 8 styles/weights of the previous Gentium Basic and to the full character set, packaged as two R B I BI families: Gentium Plus and Gentium Book 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 Basic, so are larger, but since GF subsets the difference won't affect users.
The Gentium Book Plus family is very similar but has a slightly darker weight.
To contribute, see github.com/silnrsi/font-gentium.