I don't remember why "I Have a Gun" has that name. The track is also on the second volume of "The Best of Star Trek" that GNP put out a number of years ago. If it doesn't say in the liner notes of this release, maybe it says in that one?
I just checked my copy of that album. It doesn't say anything about it. There is an overal summation of each composer's work by series ahead of the track listings, but no individual notes about the tracks themselves, beyond mentions when the Courage fanfare is included.