This will help get you on your feet with D. A tip: you can copy the code and paste it in the terminal with Shift + Ctrl + V
The ebuild you need to download is dev-lang/gdc-0.22.ebuild: http://bugs.gentoo.org/show_bug.cgi?id=48136 Download the file and rename it to gdc-0.23.ebuild.
This step is quite easy just check out http://gentoo-wiki.com/HOWTO_Installing_3rd_Party_Ebuilds
Do all the steps from the beginning to (and including) If you like living dangerously. Done!