Concerning the Origin of the Clan Gunn
...'the most probable descent of Clan Chattan is from these ancient Picts'
Page 5, MacPherson of Dalchully, The Chiefs of Clan MacPherson.
Given the great Clan Chattan descended from a tribal Pictish group it's quite logical for more clans to descend from the Picts, such as the Clan Gunn.
In my view the origin of the Clan Gunn is best seen as being that of a non-tribal group of the original inhabitants of Strathnaver / Sutherland / Caithness.
See http://clangunn.weebly.com/real-origin-of-the-clan-gunn.html for more information.