Is Sid a gym sock? He's a tube sock, but is a gym sock an entirely different garment, or simply defined by usage, like a fiddle? In the latter case, he's probably not, as I don't remember ever seeing him being worn... although I suppose that doesn't preclude the possibility of he, himself, going to the gym of his own volition.
I'm sure he washes regularly.
The Otters' medieval epic "Dim, Dim Knights" will be available soon! Watch this space!