So yeah, this is the Xturret Sentries X, made by ChoEv and was only on SFM workshop for a while despite the TurretStation 5 already got official port.

Well not anymore, as it is now finally available to Gmod.

You can find the original SFM addon here:
https://steamcommunity.com/sharedfiles/filedetails/?id=2271737785

As for the bonus weapon that came in with SFM addon, well someone already ported it, which you can find it here:
https://steamcommunity.com/sharedfiles/filedetails/?id=2390865256

Includes:
choev\xturretsentriesx\xturretsentriesx.mdl

Credits:
ChoEv - Model and Textures.
NeoLin - Porting to Garry's Mod.