I would have thought bush rustler is an endearing term for a lesbian, if anything.
Etymology isn't my strong suit.
Anyhoo, an odd thought came through my mind. Gruffudd is quite a unique name, unlike Griffiths, and in Hollywood that means something, even if people then slightly misspell it. If Alice thought he's gonna be big some day, why give the girls a wrong surname? They would benefit greatly from a name that instantly connects them to a movie star.
She just didn't want for him to have a say, didn't she? It's not about the name or the spelling or the girls, it's about Ioan leaving a mark that doesn't include her. It's about Gill liking that name and Alice showing her MiL the middle finger. It's about control.