In one of the unpublished Tolkien essays sitting around in the Bodleian library, Tolkien went into some depth about Gollum.
Gollum was according to Gandalf one of a riverside hobbit people – and therefore in origin a member of a small variety of the human race, although he had become deformed during his long inhabiting of the dark lake. His long hands are therefore more or less right.* [*Not his feet. They are exaggerated. They are described as webby (Hobbit 88), like a swan’s (I. 398), but had prehensile toes (II 219).] But he was very thin – in The L.R. emaciated, not plump and rubbery; he had for his size a large head and a long thin neck, very large eyes (protuberant), and thin lank hair . . . He is often said to be dark or black (II 219, 220 where he was in moonlight).
Gollum was never naked. He had a pocket . . . He evidently had black garments in II 219 & eagle passage II 253: like ‘the famished skeleton of some child of Men, its ragged garment still clinging to it, its long arms and legs almost bone-white and bone-thin.’
His skin was white, no doubt with a pallor increased by dwelling long in the dark, and later by hunger. He remained a human being, not an animal or a mere bogey, even if deformed in mind and body: an object of disgust, but also of pity – to the deep-sighted, such as Frodo had become. There is no need to wonder how he came by clothes or replaced them: any consideration of the tale will show that he had plenty of opportunities by theft, or charity (as of the Wood-elves), throughout his life.
(Bodleian, Department of Western Manuscripts, Tolkien Papers, A61 fols 1–31.)
This 30-page essay is mostly unpublished (and is considered a "restricted" text, which one can not gain access to without permission from the Tolkien Estate.) The preceding excerpt was printed in The History of the Hobbit.