So you saying that based on the national trust site, which only goes back to 1881, that the majority of those "common names" associated with clan gunn are in fact not? How can one tell based on information that only goes back to 1881. Clan Gunn is one of the oldest clans in scotland and by the definition of the word "sept" the names you state are "associated" with the Gunn Clan. I believe that those names you refer to are ancestors of a lineage of scandinavian (Viking) decent, regardless of geographic distribution in 1881 (well after Great Britain was in existence). Nelson, Wilson, Johnson, etc. may be common "English" or "Anglo" versions of the original name, but that doesn't necessarily mean they are not in fact of Clan Gunn, Clan Mackay (Gunn Neighbors) or any other northern highland clan. The Sept list gives us variations of names associated with the clan, not necessarily "exclusivity" to that clan. (which is not what I was saying in my original post)
I for one know that my family, which was originally Neilson, emmigrated to the new world around 1740 from Scotland due to english persicution and political strife (most likely the clearances). So any geographic "proof" in 1881 would not apply to my family, or many others either who were geographically distributed long before any "cataloging" of names was in existance. "Lore" as you stated is usually based on some kind of fact and The names Neil, Nell, Nail, Nel, and Nael are common scandinavian names. So whether it be MacNeil, MacKay, Gunn, Neilson, or Nelson, or any others you state, my bet is scandinavian decent. SO you are correct in that unless one can trace their ancestry back 500 years to the exact place and group of people, one really doesn't know, however geographic location in 1881 and 1991 doesn't prove or disprove anything.