Green’s Dictionary of Slang

gibber n.1

[? SE jib, of a horse, to back away, to refuse to go forward]

a horse dealer.

[UK]J. Poulter Discoveries (1774) 30: The Gibbers, Horse Dealers.
[Aus]Satirist & Sporting Chron. (Sydney) 8 Apr. 3/2: If put up on sale [the hounds] would not fetch a ‘tanner’ apiece ‘on the gibbers’.