The Catillac Cats never appeared in comic form- comic strips or books. They were featured in the old Heathcliff/Heathcliff and the Catillac Cats/Heathcliff Cats and Co. cartoons.
Although both Heathcliff and the Catillac Cats were set in the city of Westfinster, only Hector, Wordsworth and Mungo appeared in both segments. When the trio appeared in Heathcliff episodes, Hector was depicted as the leader of the group. Riff-Raff and Cleo never appeared in a Heathcliff episode (though Riff-Raff is present in a crowd scene cheering for Heathcliff as he fought a dog, and is mentioned a couple of times), and neither Heathcliff nor any other characters from the Heathcliff series (with the small exception of the fish store owner who did have a brief cameo in one Catillac Cats episode) ever appeared in a Catillac Cats episode. The reason is unclear, but some suspect that the McNaught company who owned Heathcliff did not want any outside characters appearing on Heathcliff. Riff-Raff and Cleo were primarily invented by Jean Chalopin and Bruno Bianchi, while Hector, Wordsworth, and Mungo were created specifically so they could appear in either series.
The only time in which Heathcliff and Riff-Raff appear on-screen together is in the end credits, when Riff-Raff snatches his hat back from Heathcliff.
Geo Gately, the man who created Heathcliff comics, had nothing to do with the Catillac Cats. Heathcliff and the Catillac Cats had two different owners.