Alias files, in gmenux2 and gmenunx, are files that tell the file selector to replace one filename with another when displaying the ROM name. Originally meant for MAME and Neo-Geo ROMS, they're also useful for use with no-intro compatible ROM collections, allowing one to have intact filenames (retroarch coverart uses nointro filenames, for example) while still having human-readable game names.
To use an alias file, find your emulator core under /mnt/gmenu2x/sections/emulators/
, and add a line to the config file
aliasfile=/mnt/PATH_TO_INTRO_FILE
. (if you're lame, you can also set it from the GUI).