Week 73! (Patreon)

Gus being a savant of sorts at making plushies was obviously revealed as part of Arc 13, the big one addressing Gus's whole thing with Ashley. It hasn't come up since then, but that's mostly due to the spotlight not at all being on Gus since that arc.

This hobby is pretty much a product of the story arc it was invented for, rather than being an inherent thing or an early addition to add personality like the other characters'. There isn't a whole lot to say about why Gus has this strange ability to make really top-quality stuffed toys. You could say it's just a statement that sometimes people are just really good at a certain skill without having any training or experience, I suppose. It also gives Gus something else that he can dedicate time into doing besides, well, stalking Ashley all the time, not that he would be doing that anymore after that whole arc anyway.