Well, just a Shambler shotgun model from Metro Exodus, now ported to Gmod.

You can find the original SFM addon here:
https://sfmlab.com/project/29155/

Includes:
minitiv\metroexodus\weapons\shotgun_uboinik_shambler.mdl

Credits:
Deep Silver/4A games - Metro Exodus
Minitiv - Porting to Source Filmmaker.
NeoLin - Porting to Garry's Mod.