He won the fight by unanimous decision, with scores of 30-27, 30-26 and 30-26. Gunji was scheduled to face the reigning Krush Featherweight champion Takahito Niimi at K-1 World GP 2021: Yokohamatsuri on September 20, 2021. Gunji won the fight by a comfortable unanimous decision, with two judges scoring the bout 30-29 in his favor, while the third judge awarded him all three rounds of the fight. K-1 Featherweight champion Gunji was scheduled to challenge the reigning K-1 Featherweight champion Tatsuya Tsubakihara at K-1 World GP 2021 Japan on December 4, 2021. Gunji and Tsubakihara had fought twice as amateurs and twice as professionals, with Gunji earning the victory on two occasions. Gunji won the fight by split decision, after an extra round was fought. The fight was scored a split draw after the first three rounds were fought, which meant an extension round would be contested. He edged Tsubakihara in another close round, with two of the three judges awarding him a 10-9 scorecard. Gunji was booked to face Toma Tanabe in a non-title bout at K-1: K'Festa 5 on April 3, 2022. According to Gunji, he originally intended to defend his title against a foreign opponent, but accepted a non-title bout against Tanabe as foreign competitors weren't able to enter Japan due to the COVID-19 restrictions imposed by the Japanese government. At the weigh-ins, Gunji missed weight, but was able to make the featherweight limit in his second attempt. The fight was ruled a majority draw following the first three rounds, with two judges scoring it an even 30–30 and 29–29 draw, while the third judge scored it 30–29 for Toma. An extension round was therefore fought, after which Gunji was awarded the unanimous decision. Gunji faced the WGP Kickboxing veteran, and the promotions former champion, Facu Suarez in the quarterfinals bout of the 2022 K-1 Featherweight World Grand Prix, which was held at K-1 World GP 2022 in Fukuoka on August 11, 2022. Despite a slow start to the fight, which saw one of the judges score the opening round as a draw, Gunji would go on to dominate the remaining two rounds en-route to winning the bout by unanimous decision. Two judges scored the fight 30–27 for him, while the third judge scored the fight 30–28 in his favor. Gunji advanced to the penultimate bout of the one-day tournament, where he faced the reigning Krush Featherweight