Go to Quincy ruins. Save the game cause you'll get rekt hard up the rear. Kill stuff that moves, and WATCH THE FUCK OUT FOR THE CHURCH. Basically, some dude is wearing the armor you want, but he has a rocket launcher. So kill stuff while dodging rockets, then go up and murder that peasant.
I did this before lvl 20. Wich is hard, i'll admit. However, if you do it later, you got the sweet company of stealthed oneshot assaultron domminators.
Technically the institute/synth armor is better, but heavy combat armor is like the most rare in game i've seen, and looks awesome. It lacks one of the arms, but youll drop that later somewhere.
For the Gaus rifle, on the bottom left of the map, the permanently radiated zone, once you clear the normal enemy's, legendary radscorps, deathclaws etc will spawn frequently when revisiting. Take the Hazam suit and your shotgun and murder them. Having rifleman , crit banker, 4 clover, grim walker, kinda helps ALOT there, or watch your enviroment well and take it one at a time