The gun doesn't appear on Gmod addons list (and I mean in ALL the lists). That is probably due to the fact that the gun is too heavy about vertices, but to fix it is really simple: it would be necessary just split up the gun in two models (and in the game, all you need to do is just bonemerge both parts to have the whole gun).

Uh. that's Weird.
because I've tested multiple times in steam AND with friends with low/high-end pcs.

None of them didn't have problems with spawing it.

I guess if there's more problems i'll see more into it.

I'm re-trying to download it, and see if it's just a coincidence.
All right, I'm back.
So, I downloaded the model outside steam and I started Gmod again.
The model appears on list JUST if the model is downloaded outside (and yeah, I tried again to do the sub and unsub thing, but same result as earlier) steam.
It appears as effect, but also Gmod "sees" it as static prop (meaning: has just one bone).
EDIT: I forgot to mention it works even subbing it, but still, it appears ONLY as static prop, and not as prop with bones.