So I have found Military Fatigues and Combat Armor but I don't know where to find Heavy Combat Armor. I have looked at the places that Gunners spawn, but I can't find Heavy Combat Armor. I have the Sturdy Combat Armor and the normal Combat Armor, so I don't need those. Do you get Heavy Combat Armor in a mission or something?
While there is no guaranteed full set of Heavy Combat Armor, there is a place that you can definitely find every piece of the Heavy Combat Armor set minus the Right Arm. From the combat armor wikia page:
Left arm, left leg, right leg and chest piece of heavy combat armor can be found within the Quincy ruins on a named character or, if the character has already been killed, a Gunner sergeant found on the church roof. A right arm heavy muffled combat armor may be found on the character Cruz, after killing him, inside the Gunners plaza building. It is not guaranteed to be hardened though.
In case the right arm you encounter there is not Heavy, try this (from the same page):
Alternatively, one can buy the heavy combat armor right arm and the left arm in Vault 81.
As DCShannon mentioned in a comment, Gunners will start wearing/dropping Heavy Combat Armor after a "certain level," but unfortunately, the wikia is not clear on what level that would be.
You cand find a full set minus arm at:
-hubcity car wreckers, the gunner boss on the overpass will wear it. He wil spawn with a fatman, if you get more near, he will start shooting at you with an high lever auto laser gun (overcharged auto laser/laser minigun) in the control room, where you need to use the crane, you also can find a combat helmet.
-quincy, baker will spawn on the top of the church wielding a fatman. I killed hin from far away with an anti material sniper rifle from far away(my tip). On the overpass there will be another gunner boss called Clint, wearing a T60 PA torso and T51 right+left arms and wield a laser minigun. In the back of the city the third gunner boss (tessa) spawns. He will wear an T51 Torso and his legendary raider right arm. At higher levels he can spawn with other Raider-PA parts.
In nula world in the star control you can find a T51F helmet in the star core room at the second floor. So you can have at least one full set of T51.