As i understand it, that prop was simply removed from the map entirely for some reason. Interesting.

ok i since fixed it, it wasnt a missing model but for some reason source was calling no model instead of the error model or a replacement, in human language, invisible
for some reason an addon made source do this, deleted of a bunch of addons and it fixed itself, the same issue was actually happening to regular source games before even gmod was a thing, i found a conversation on here about just that problem happening in hl2 from almost 20 years ago

This truck is from HL2, and since GMod already has HL2 mounted in, it should be visible without any addons or third-party games.
The problem is on your side.

truck is completely invis wtf

w map