In the reference books I have consulted, the spelling for the word that means "stranger" under
Fingal is
gall, with two l's, while the word that means "valor" is
gal, with only one l. I don't know why such a minor difference in spelling should be associated with differences in meaning back in medieval times when spelling was not set, but perhaps the difference between one and two l's is indicating that there was a subtle pronunciation difference in Gaelic between the two words. Certainly in modern English the spellings "gall" and "gal" are pronounced differently. :)