Fox Deportes first reported the news and UFC officials confirmed the match-up on Thursday.
Gastelum (7-0) won the seventeenth season of the popular reality show by defeating Uriah Hall by split decision on April 13. In his second UFC fight and his welterweight debut, Gastelum defeated Brian Melancon by submission in the opening round.
Gastelum hopes to keep his undefeated record intact and extending his winning streak to eight.
McGee (16-3) won the eleventh season of The Ulitimate Fighter by defeating Kris McCray via submission in the second round in June 2010. McGee won his next two outings, but after losing back-to-back fights to Costa Philippou and Nick Ring, he decided to drop from middleweight down to the 170-pound division. Since changing weight classes, McGee is on a two-fight winning streak with victories over Josh Neer and Robert Whittaker.
UFC on Fox 9 takes place on Dec. 14 at the Sleep Train Arena in Sacramento, Calif. A lightweight title fight between newly crowned champion Anthony Pettis and challenger Josh Thomson headlines the event.