Gastelum won a 5A Division II AIA state wrestling title out of Cibola High School[4] and continued to wrestle at North Idaho College, where he was ranked as high as fifth in the country (NJCAA) as a freshman before deciding to compete in mixed martial arts professionally.
[5][6] After turning professional, Gastelum amassed an undefeated record of five wins (all finishes) and no losses before competing in The Ultimate Fighter.
[13] Gastelum won the fight via split decision, becoming the youngest winner in the show's history at 21 years old and earned a UFC contract.
After the victory, Gastelum announced he would be moving down a weight class to compete in the UFC's welterweight division.
[17] Gastelum was expected to face Court McGee on December 14, 2013, at UFC on Fox 9, but was forced to withdraw due to a torn PCL.
In the post-fight interview with Joe Rogan, Woodley said that he would not accept 30% of Gastelum's purse, as him missing weight and losing was enough punishment.
[31] Gastelum was expected to face Matt Brown in a welterweight bout on November 21, 2015, at The Ultimate Fighter Latin America 2 Finale.
[41][42] This made it the third time Gastelum failed to make the welterweight limit, prompting Dana White to say that he will "never let him fight at 170 again".
[45] Gastelum next fought Vitor Belfort on March 11, 2017, in the main event at UFC Fight Night 106.
[49] However, he was pulled from the event after USADA learned he tested positive for high levels of marijuana metabolite Carboxy-Tetrahydrocannabinol (Carboxy-THC) above the 180 mL allowance by the World Anti-Doping Agency (WADA) from his previous drug test for his bout against Belfort.
[50] On May 7, Gastelum's win over Belfort was officially overturned to a "No Contest" and he was suspended 90 days (retroactive to the March 11 test date).
[53] The bout with Anderson Silva was rescheduled and was expected to take place at a UFC Fight Night on November 25, 2017.
[54] However, Silva was pulled from the bout due to failing a USADA drug test and was replaced by Michael Bisping.
[60] In July 2018, the UFC announced that Gastelum and Robert Whittaker were selected as coaches for Ultimate Fighter 28.
[62] However, Whittaker pulled out of the fight a few hours before the event and was forced to undergo emergency dual surgery later in the day due to an abdominal hernia of the intestine and a twisted and collapsed bowel.
[69] Subsequent to the event's weigh-ins, the NYSAC issued a rare statement that they would be pursuing disciplinary action against Gastelum.
As the result, Gastelum and his coach, Rafael Cordeiro, were fined $1,000 and $200 respectively by New York State Athletic Commission.
[78] Gastelum faced Robert Whittaker, replacing Paulo Costa, on April 17, 2021, at UFC on ESPN 22.
[84] However, Imavov was forced to withdraw due to visa issues[85] and was replaced by Dricus du Plessis.
[86] However, Gastelum withdrew a week before the event due to an undisclosed injury and his bout with du Plessis was canceled.
[88] However, the week of the event, Gastelum withdrew due to a mouth injury and was replaced by Sean Strickland in a light heavyweight bout.
[92] Gastelum was scheduled to face Shavkat Rakhmonov in a welterweight bout on September 16, 2023, at UFC Fight Night 227.