All gnat stuff is heavily broken and is going to be removed, hence, this optional support needs to be disabled here Thanks
Completely removed the support for ada in plplot. commit 077fe42177f6a0a36edf437b6fd3e49bf7d590e8 Author: David Seifert <soap@gentoo.org> Date: Sat Oct 8 14:11:11 2016 +0200 sci-libs/plplot: Remove support for USE="ada" Gentoo-bug: 596530