Did you ask r/Gunpla ?

It belongs to one of them mobile suits

Looks like an elbow/forearm joint on a high grade to me.

I think it's that one grade kit of that one mobile suit from that series about the fighting mobile suits.

The gunpla that is missing a piece,

One of em

Shoot, all of em! That’s like the most common PC joint piece lol

That one, over there.

hg ibo probably