Is suicide by gun a "misuse" of a gun?
You have every right to end your own life. In some cases- very few- I can even imagine it being the best thing to do.
You have a right to use your own tools to do what you do, as long as you don't violate anyone else. So, don't commit suicide on other people's property and be aware of what's beyond your target. And make sure the gun is unlikely to be picked up and used by an irresponsible person after you are done with it. No, don't consider this a lesson in "how to".
But, to say using a gun to do what you have a right to do, and even to kill someone who is a credible threat to your person or property- which you could be- is a "misuse" of a gun... I'm not sure about that. It seems like that's a case of using it in a way it is designed to be used.
The people who don't want you to kill yourself might disagree, of course. Your death would probably make them extremely sad. Do they own you? Does the State own you? Who has a prior claim* on your life?
In any case, I don't think anti-gun "statistics" should count suicide as a "misuse of a gun". But I know they will continue to do so. Dishonestly.
*Sure, some religions would claim a deity owns you, but if that's the case that deity has a responsibility to keep its property well cared for- even if the property does stupid things and causes its own problems. If your dog gets tangled up in its collar through its own stupidity, it's still your responsibility to rescue it and fix its injuries, isn't it. I know- it's standard procedure to excuse the deity for anything and put the responsibility for the problems back on the human. Which kinda brings us back to the beginning of this circle. And anything which just takes you in a circle can probably be safely ignored in this current case.