Gabby Forrester is one of the seven Forrester brothers. He is described as "shorter than the rest and not unduly bright." Not a lot is mentioned about him through the course of the book.
The Forrester brothers include Buck, Mill-wheel, Gabby, Pack, Arch, Lem, and Fodder-wing. Lem is the meanest of the bunch, while Buck is the most intelligent and the one who is the most friendly to the Baxters. Fodder-wing is mentally and physically challenged, but is beloved by them all.
Gabby appears briefly when the Forresters help out after Penny is snake-bit. Buck sets out for the Doc, and laments that there is not "a drop o' whiskey" for Penny. Gabby, who appears to have a bit of a wry sense of humor, says,
"Ol' Doc'll have some. If he's purty tol'able sober, he'll have some left. If he's drunk all he's got, he kin blow his breath, and that'll make a powerful portion" (Chapter 14).
Gabby is also mentioned by name when, in the aftermath of the great storm, the Forresters and the Baxters find a batch of bear cubs, saving them from marauding wolves. Gabby and Mill-wheel stay behing to watch the cubs while the others return to the Forresters' place to fetch a wagon in which to bring them home (Chapter 24).
Gabby appears to have a kind heart. When Rip is hurt while fighting old Slewfoot, it is he who steps up to pick the dog up and carry him in the saddle in front of him (Chapter 26).