Upstream released SIL Gentium 6.000 and 6.001 recently.
The change is huge: in version 5 we only had Regular and Italic versions, but version 6 has also Bold and Bold Italic ones. On the other hand, the "compact" variant of the font, which used to be made available via a USE flag, seems discontinued.
The documentation structure is also vastly changed.
I created a tentative ebuild that reflects the abovementioned changes, which might be used as a reference: