I have to say, with no evidence to back it up, that if you want to die enough to put a barrel in your mouth, if that gun is taken away, you can find another way.
We have to be better than harm reduction surely?
Thing is, often people who are depressed do get better. And I think a lot of people who end up killing themselves would have gotten better if only they hadn't. I think this is all the more the case for teenagers.
That's why I think it's worthwhile to take away opportunities to kill yourself. That means making gun possession illegal, installing safety nets, etcetera. (Within reason, of course.)