I recently learned about the general MIDI standard and one thing that I cannot understand is the last sound effect is always a gunshot. Most of the other effects like applause, telephone ring, and bird tweets make sense and I've heard in several different songs, but I don't think I've ever heard a gunshot used in a song before. How did the creators of General MIDI standard decide on what sound effects to include in the standard?
According to the MMA,
Roland was one of the early proponents of GM and proposed that the GM Sound Set include sound effects for use with games, as was the case with their CM32L sound module.
I can't comment on why the committee decided on those particular sounds as I wasn't there, but I will say that gunshot sound effects are very common in musicals, and until recently it was common for any kind of timing sensitive sound effect to be in a synthesizer book. Now we have laptops and software like QLab so it's more practical for it to be fired from the sound booth, but those older shows are still around.
The MIDI Manufacturer's Association released the General MIDI Specification for hardware sample players and sample-based keyboard synthesizers in 1991. Read about it at the Wikipedia article, which includes reference links to the actual published specifications.
I do not think there is a definitive answer to your question. I believe the industry committee wanted to define 128 standard instrument sounds covering the basics for pop music and orchestra, and then they filled up the remaining slots with some amusing sound effects to help promote the idea that this was another use for the newly-affordable technology of hardware ROM (read-only memory)-based sample players.
Pure speculation on my part but from what I know about the 555 chip a "gunshot" is a pretty easy sound to make with a basic electronics setup. After a siren it's one of the first sounds people make when they're learning how to create sound effects using electronics. For instance: http://www.555-timer-circuits.com/machine-gun.html