"above" is an adverb and can't be used as an attributive adjective before a noun. So
"the above gun" is wrong grammar.
"above" is also a preposition. See OALD:
There is even an entry about above as adjective. One special case, in letters, e.g.
"the above person".
As this is short for the above mentioned person, I would not say that it is adequate to lable "above" simply as adjective. At least a remark would be necessary,
"above, attributive adj., limited use".