I lowered the MLB debut age to 23, and changed the criteria to be 10+ starts in the first or second season. This produced the following list of 41 guys that includes Gio:
Aaron Harang
Aaron Heilman
Andy Larkin
Angel Guzman
Blake Snell
Blake Stein
Brandon Webb
Carlos Marmol
Daisuke Matsuzaka
Dan Reichert
Dan Wright
Daniel Cabrera
Doug Davis
Eric Gagne
Gio Gonzalez
J.D. Durbin
Jarred Cosart
Jesus Sanchez
Joaquin Benoit
Joe Nathan
Joel Hanrahan
Justin Miller
Kazuhisa Ishii
Kerry Wood
Kip Wells
Kyle Drabek
Micah Bowie
Octavio Dotel
Paul Rigdon
Radhames Liz
Rafael Medina
Randy Keisler
Rob Tejeda
Russ Ortiz
Ryan Glynn
Scott Eyre
Scott Williamson
Sean Newcomb
Seth McClung
Tony Saunders
Victor Zambrano
A cursory review of the list reveals 4 good SPs (Gio, Harang, Wood and Webb), a handful of good BP guys, a few older foreign imports, a few mediocre guys like Russ Ortiz, and lot more busts.
So still about a 10% chance to be an impact SP.