cdinstall - Copy files from the CD rather than asking the user to copy them, mostly used with games
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
duke3d - Install the Duke Nukem 3D data files, provided by games-fps/duke3d-data.
flac - Add support for FLAC: Free Lossless Audio Codec
fluidsynth - Install media-libs/sdl2-mixer with fluidsynth support.
gtk - Add support for x11-libs/gtk+ (The GIMP Toolkit)
hrp - Install the Duke Nukem 3D High Resolution Pack.