as has been said, there is a type of air pistol, generally mid range powered for one of that sort, usually used to kill mice and rats, just shrot of the power of a plinker. But also the terms marker and gat are used to avoid the word gun. Contrary to what has been said, the military do not use the word gun, they have rifles, pistol, automatic rifles etc etc. Just what i was taught but am lead to beleive by several sources this is the closest definition.
Another possibility is that the word gat has nothing REAL to do with firearms of any sort and it is a word that is mutually understood by people as with markers. Hope this helps
cheers, sTevie::C