I think that it all depends on what each Clan considers to be a "gentlemen of the Clan". Is it enough to be a recognised gentleman or would the Clan require that the person be a land owner (in a designated area) in their own right. Or maybe even a second generation land owner, to prevent incomers having too much influence.

An Army Officer, by virtue of their holding the Queen's/King's commission, is considered by Lord Lyon and the College to be a gentleman and would in the normal course of events be granted arms if they applied.