Gun is my choice and I have a collection of them (I love to shoot). But it would be easy to act on impulse because it is available and quick, no planning required.
I did an internet search once for pictures of people who have done this. In my work, I saw a lot of blood and guts, became almost immune to it. But after seeing those pictures, I had serious doubts. It may not be for you.
I also have been involved in the care of a few failed suicide attempts. Trust me, it's not a pretty picture and this is one of the top reasons I have not attempted (yet). There is no calling someone to save you once you pull that trigger. There is no changing your mind. It is final, Very Final. All done.
As for using your father's gun, I have mixed thoughts about that. Do you know how to use it? This is a method you do not want to screw up! The legal ramifications for him, the potential for lifelong guilt, may or may not matter to you? Perhaps that is one of your reasons, to get even with him? To teach him a lesson? If he is a bad person, would he even care? I have thought all of these thoughts myself. It's a vicious circle.