One can be found on a dead Pitt raider in an irradiated area to the left of the entrance to the steelyard, as you face away from the door.
Notes[]
In the Effects menu, the radiation resistance and Agility modifiers are marked as coming from "Lead-Lined Armor", not gamma shield armor. This may explain the extra weight, as the gamma shield armor may shield against gamma radiation through the use of a lead lining, hence the description as lead-lined armor.
Behind the scenes[]
There is a leveled list for the gamma shield armor defined in the G.E.C.K., but it's not used anywhere, so the gamma shield armor will not appear in any random loot.
Bugs[]
This armor tends to "phase" once dropped: it will disappear after a while if dropped, turning completely invisible, while it will still actually be there and can be interacted with/picked up.