rxvt-unicode with patches for glyph support (Powerline fonts, Nerd Fonts) and truecolor support
Copy x11-terms/rxvt-unicode directory in your local overlay and emerge rxvt-unicode
You should have this line in package.accept_keywords :
x11-terms/rxvt-unicode ~amd64
If you want glyph support wide-glyphs USE flag should be enabled
If you want truecolor support enable both 24-bit-color and 256-color USE flags