Hi Sakaki, is there a D language compiler for Gentoo64, I saw nothing in your repo?
Will doing this work or will Layman break my Gentoo64?
Haven't tried building this yet. Installing layman shouldn't break anything, but probably better to use the "eselect repository" approach instead nowadays (both ways are provided in the dlang Gentoo wiki page you linked).
Yes, you can also compile sys-devel/gcc with e.g. the objc and objc++ USE flags, but I try to keep the shipped version of gcc relatively vanilla.