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.
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.
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.