To leapfrog the cynical point of view. Yes, I believe that the Galloway tartan would be a perfectly legitimate choice for you. You could even go with Ayrshire as your family name seems to be well concentrated there as well. As an American without a clear clan connection you are on the right track choosing a tartan that has some historical connection to your name and would have some personal meaning to you. The choice is yours, is a personal one, and if I were you I'd be proud wearing either.