How do you get the Minigun, Gatling Laser, ect., and ammo for them?
You can get these weapons by killing Super Mutants who are carrying them. You can also get some ammo for these off of dead Mutants. If you want, you can buy the Minigun at the Gunrunners place, and you can get a Rocket Launcher from Jake in Oldtown of the Hub. I think there may be a place where you can get a free Rocket Launcher (maybe from the Gunrunners, from one of the lockers that are blocked by guys. After you have defeated the Deathclaw and brought the good news to Gabriel, the people blocking the way to the lockers should move, and then have complete access to them. If I remember right, there should be a Rocket Launcher in one of them.) There is no place in the game where you can just pick up a Gatling Laser from a shelf or out of a locker. You have to kill a Mutant who has one in order to acquire it. If you go to the Children of the Cathedral church, you can get one off of a Mutant there. Just enter the back hall behind the big screen. There is a Mutant guarding the staircase.