Eight years on, the question of what Gilbert actually is has gone underexamined. The cats are the cats. He is Gilbert. The taxonomy has not been pressed for further detail. With the archive now sorted by coat — ginger over here, brown tabby over there — the description is owed.
Pattern
Gilbert is a classic tabby — sometimes called blotched, sometimes marbled. The distinguishing feature is the wide, swirled pattern on the flank, often with a bullseye where a more striped (mackerel) tabby would have narrow vertical lines. The bullseye is plain in the photograph below: the dark markings broaden into a swirl across the side, rather than running in parallel stripes from the spine downward.
Colour
The base coat — the agouti banding between the dark markings — is warm brown and tan, not silver and not grey. The stripes themselves are very dark, almost black in shadow, but Gilbert is not a black cat: the nose leather is pink, and a true black-tabby would carry a black nose. This is brown tabby, the most common colour in the species, the wild type the rest of the patterns are mutations of.
White markings
An inverted V on the face, a white chin, a white chest and belly, and four white socks. In cat-fancy terms this is a bicolour of the low-grade variety — white covers the underside and feet but does not reach over the back. The face V is a tidy thing, neither asymmetrical nor incidental; it gives Gilbert the expression of a cat about to say something just short of out loud.
What Gilbert does
The catalogue, abbreviated. He sleeps on the white bedding when permitted, curled into the kind of comma a body makes only after a great deal of unhurried thought. He watches the world from the windowsill, with the air of a magistrate. He holds the long stare of a cat on alert at the back gate in the dusk. He marks the landing bannister with the side of his face, as cats have always done. And he uses the scratching post, occasionally, when there is no nearer chair leg.
The verdict
Brown classic tabby with white — bicolour, low-grade. Not a torbie (no ginger patches anywhere on the body, only the warm cast of the agouti coat in sunlight that initially suggested otherwise). Not a black cat, not a grey tabby, not a mackerel tabby. A common coat, beautifully assembled, on a cat who has spent eight years declining to explain himself.