The latest development snapshot is 0.45.3 - February 12/2019 -- The latest stable version is 0.44.0. - June 24th/2018 new $HOMEPAGE: https://www.megamek.org/
This package migrated to Gradle since 0.45.2, at best it'd make sense to add old stable 0.44.0 but beyond that can't easily work on Gentoo right now.