Coupla things:
First, it's not just GURPS which spells it with two "l"s. The Myth manual does as well. There seem to be variant spellings within the Myth canon. So it's probably a lost cause to try to argue for one as more "proper" than the other.
Second, I know of at least a couple of (reputable) dictionaries which allow Leveler and Leveller to be entirely synonymous. Whether or not this was a mistake originally, it's so common now that it's simply a part of the English language. (Though its frequency varies from region to region.)
Best not to worry about it, I think. Thanks for the history nugget, though. Myth etymology is always fun. :-)
--SiliconDream
