In order to build an android app, I need gradle.  Apparently, there's
only a binary version in gentoo, dev-java/gradle-bin, but there's a
source version in the mva overlay.  Why might it only be in an overlay?

This link:

https://wiki.gentoo.org/wiki/Gradle#Availability

links to:

https://github.com/msva/mva-overlay/tree/master/dev-java/gradle

containing 3 files, none of which is 100 lines.

If that's all it takes, why not be part of the distribution?

TIA