From publisher blurb:
One of the races considered prominent in Mitgarthr is gnomes, but for those who like to play their Labyrinth Lord games with only the core rules there are no rules to play them!
Mithgarthr Entertainment presents our version of a race-as-class gnome! Inside you'll find a description of thier appearance (notably different than that given for gnomes in the monster section of the core rule book; these are Mithgarthian gnomes!), ability descriptions, leveling charts, and all that fun stuff.
And what sets them apart from the other race-as-class fellows in the core book? The Mithgarthian gnome allows you to have a Theif/Magic-User (akin to an elf being a Fighter/Magic-User). How fun is that?