I have found that this is a term mostly used by the "sensible regulations" crowd. It seems an odd term to use, why not gun murders or fire arm homicides? The reason is that; of the usual 30,000/year figure given, 8,600 are homocides and the remaining 21,400 are suicides. (You can get to these figures either from published statistics on suicide rate or from FBI crime statistics--all easily obtainable on the web).
These sites almost never point-out this breakdown, so I can only assume their intent to deceive. But still, a death is a death and tragic whether cis (suicide) or trans (homicide). Surely, guns play some role in both, given that most (8,600 out of 12,700) trans and about half of cis killings use a fire arm.
There is certainly a factor of utility: A murder victim is likely to resist and a gun makes the outcome of the encounter more certain. Similarly, a gun is a logical choice when considering cis killing:
1. It is certain.
2. It is fast.
3 It is painless
Most other choices fall down in one or more of these categories. Take hanging for instance; it might not work, it will be both painful and slow. The only drawback I could see to shooting is that it leaves a mess. I make no claim to mental health insight, but it seems like cis killers do often want to lash out at those they leave behind. So, a mess may be more a feature than a bug.
Guns help if you want to kill yourself and so, logically, the availability of a fire arm would make it more likely that a given person would go through with it.*
To see if there is a correlation between the availability of an easy cis killing method and actual cis killing, we can compare suicide rates with other countries which have much more restrictive gun laws than we do here in the USA:
Japan is often held-up as a great example of how a low murder rate correlates with gun restrictions. Indeed, their rate of 0.4/100,000 is much better than our 4.8/100,000. On the other hand, their suicide rate is 21.9/100,000 to our 12.0/100,000! So clearly, the lack of guns is not sufficient to stop people from cis killing if that is what they want to do.
*I will note here that asphyxiation with an inert gas would be ideal in terms of the above three consideration and have the virtue of being less messy than a gun shot, but it is not a method which is well known. It seems that the feeling of suffocation you get while holding your breath is due to CO2 build-up in the blood causing a drop in pH. Breathing an inert gas allows the CO2 to be driven-off.
Added: Tom Maguire points out that if the problem is 30K "gun deaths" and most of them are suicide--then isn't the problem mostly a mental health issue?