Gabby was originally introduced as the nearly-identical son of Johnny Gabrielli, one of the founding members of the original Newsboy Legion. He was later revealed to actually be a clone of his father, created by Project Cadmus as part of an abortive plot. Following in the steps of his surrogate father, Gabby joined the new Newsboy Legion with the clones of the other original members.
The Pre-Crisis Gabby was created by Jack Kirby.
The Post-Crisis Gabby was created by Roger Stern and Ron Frenz.
Initially simply the son of the original Johnny Gabrielli, Gabby was eventually revealed as a clone who had been aged up to the age his surrogate father had been when he was first involved in the Newsboy Legion. As such he shares his personality with his surrogate father, and he has all the memories of his father, but only up till the age to which he has been artificially aged. He and the Legion worked with Cadmus, but eventually left the Project along with their surrogate fathers. They also aided Kon-El and Jimmy Olsen at different points in time. His fate is unknown following the death of his surrogate father.
In Other Media
Gabby appeared in an unnamed cameo role along with the Newsboy Legion in the Justice League Unlimited episode "Patriot Act", where he was given the job of crowd control by Vigilante, and threw debris at General Eiling along with the rest of the Legion. Though it's never stated in the episode which incarnation of the Newsboys the four that appear in this episode are meant to be, hints in the dialogue ("[our parents have] run off I guess") and the fact that Cadmus already exists in the DCAU suggests that this version of Gabby is the clone.