What is this?
An unofficial overlay obviously, to experiment with ebuilds.
To manually add overlay:
layman -f -a gentoo-overlay-trent -o https://raw.githubusercontent.com/TrentSPalmer/gentoo-overlay-trent/master/overlay.xml
Here's a Gist with instructions for how to quickly spin up an nspawn container to do some test building, if you are into that sort of thing.