The two middleweight finalists, both from coach Chael Sonnen's team, will meet Saturday night in Las Vegas in the live finale of the reality TV show.
The winning fighter gets a contract to fight in the UFC and a Harley-Davidson motorcycle. Others from the cast will likely follow him given the UFC considers the TV show to be its minor league pipeline.
In the taped episode that aired Tuesday night, Hall stopped Dylan Andrews by second-round TKO while Gastelum submitted Josh Samman. Both losers were from coach Jon (Bones) Jones' team.
The two coaches will meet at UFC 159 on April 27 in Newark, N.J. By virtue of sending two fighters to the final, Sonnen also wins a Harley-Davidson.
Hall was the third fighter picked when the coaches chose teams. Gastelum was the penultimate at 13th.
Hall, 28, has been a freight train all season.
The Jamaican-born New Yorker had two highlight-light reel stoppages, knocking out Adam Cella with a spinning heel kick and flooring Bubba McDaniels with a punch. Both losers were on the canvas for a while.
"This kid is the nastiest, deadliest, meanest, kid that we have ever seen in the history of TUF,'" said UFC president Dana White.
"You are a contender for the (UFC) title," Sonnen told Hall after his quarter-final win over McDaniels. "I spent five years in that weight class and you're a contender."
Andrews, the last fighter picked, came forward at Hall in a first round fought entirely on the feet. But Hall scored more and had the heavier kicks and strikes. Thanks to Hall's jab, Andrews had a mouse under his right eye and a lumpy face by the time the round had ended.
Hall rebuffed a takedown attempt in the second. His punches continued to take a toll, bloodying Andrew's face.
Andrews finally got Hall down about half through the round, but Hall worked for a kimura submission from the bottom and then starting striking from below. He suddenly reversed position and hammered down blows on Andrews until the referee stepped in with seconds remaining in the round.
"My heart hurts more than my face does," said an emotional Andrews, a New Zealand native who lives in Australia.
At 6-3, Samman had five inches in height and seven in reach on the 21-year-old Gastelum.
But Gastelum had an advantage on the ground and made the most of it in the first round. That led to a rear naked choke, with Samman never really getting his game going.
"I'm just in shock right now," said Samman, the fourth pick who had not been short of confidence on the show.
Arizona's Gastelum advanced to the final with a submission win over McDaniels, who returned to the competition via a wild card after losing, and knocked out Collin Hart.