I haven't used it yet, but Bored of the Rings' Weapon Welder apparently will do the conversion of FPSC weapons to be used in GG. He has a free version now available in the Third Party Tools forum:
https://forum.game-guru.com/thread/218121?page=1#msg2580110
EDIT: I just looked at it and I think it only work for the player's weapons, not NPC weapons. I could be wrong, though. I'm sure someone will be along here eventually and solve your issue. I wish you luck!
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