Compounding the problem is that there is no chief of the Gunn clan (so of course it isn't even really a clan, according to the accepted notions of what constitutes a clan), who would normally be the authority that decides which surnames and specific families are septs and which aren't.
Slightly off-topic, but I did hear tell that Iain Gunn of Banniskirk, the Commander of the Clan Gunn, was recently given a seat on the Standing Council of Scottish Chiefs.

Todd