In the lore surrounding TLW and Thorn, we see that in the final showdown between Malphur and Dredgen Yor, Malphur hits Yor with 3 shots, and he describes feeling powerful and full of light. He is also frequently described as extremely powerful and special. This, coupled with the fact that TLW and golden gun's models are identical in shape and size, as well as the overt amount of lore surrounding both guns, suggest to me that Malphur was possibly the first to use the Golden gun.