It's an excellent tool for the job. There are other methods as many celebrities continue to prove. My Plan B is really good drugs and a cold night, but Plan C is a .45.
Here's the other thing...
The success rates for suicide attempts with drugs, hanging, or other forms of self-harm are much lower than the success rate for suicide attempts with guns.
So even if you tried to assume that all 25,000 of those gun suicides would be other suicides, you couldn't really count on that full 25,000 number because suicide attempts by overdosing, hanging, or slitting your wrists sometimes -not always- but sometimes don't work.
So removing guns from the picture would certainly reduce that suicide rate by a ton.