Here's what ultimately became of the challengers in the 2004 Galarian Gym Challenge.
When Galar's 2004 Gym Challenge commenced there were 63 trainers competing in the challenge, but by the time of Marnie's murder this had been reduced to 19 (Marnie included). Nearly all of them joined the rebellion:
Gloria Bell - A girl from Postwick. She joined the rebellion early on and proved herself one of the GNLF's most determined and brave fighters, being the one to deal the final blow against Eternatus during the Darkest Day. She was awarded Galar's highest military honour by Alexander for her service, and would ultimately go on to defeat Leon in the 2005 Gym Challenge and become Galar's second Champion since independence, a title she would hold until 2019.
Hop Lockhart - A boy from Postwick and Champion Leon's younger brother. He was instrumental in convincing Champion Leon to defect. Surviving the rebellion, he would later become a Pokémon professor and Gloria's wife.
Victor Bell - A boy from Postwick and Gloria's twin brother. He would serve in the Battle of Motostoke and after Kabu's death would serve as Motostoke's military governor and subsequently its Gym Leader.
Marnie Maple - The girl from Spikemuth whose death set the rebellion off. She went missing shortly after defeating Gym Leader Gordie at Circhester and would be found dead four hours later.
Yue Baijin - A girl from Stow-on-Side. She went missing during the battle for her hometown and is presumed dead.
Polaire Postpich - A boy from Wedgehurst. He was killed in action during the Darkest Day.
Kent Jordan - A boy from Motostoke. He served in the Battle of Hammerlocke, but Rainhan's Duralodon managed to inflict a severe abdominal wound that he died of complications from.
Cher - A non-binary person from Hammerlocke. They survived the rebellion unscathed and would go on to become an LGBTQIA+ rights activist in Galar./li
Deneb Cooke - A girl from Ballonlea. She was killed in action during the Battle of Wedgehurst.
Icla Sutton - A girl from Spikemuth. She was killed in action during the Battle of Wyndon.
Wei Baijin - A boy from Stow-on-Side and Yue's younger brother. He survived the rebellion but lost his leg during the battle for his hometown. He has since become a major disability rights advocate throughout Galar and the nearby region of Paldea.
Izar Kelly - A girl from Turffield. She died of injuries sustained from getting hit with shrapnel from a government shell during the Battle of Hulbury.
Dunne Hughes - A boy from Hulbury. He was killed by shrapnel from the same shell that killed Izar.
Pia Peterson - A girl from Circhester. She was killed in action by friendly fire during the Battle of Motostoke.
Corvin Cage - A boy from Wyndon. He died bleeding out from injuries sustained during the Battle of Wyndon.
Terry Wallace - A boy from Wedgehurst. He survived the rebellion but was left permanently deaf. He moved to Sinnoh following the rebellion.
Theemin Byrne - A boy from Hammerlocke. He survived the rebellion but was left with digestive issues (injuries he sustained from a shell necessitated the removal of his stomach and half of his small intestine).
Phoebus Fisher - A boy from Ballonlea. He was killed in action by Nessa's Drednaw during the Battle of Hulbury.
Vega Violet - A girl from Circhester. She is notable for being the only Gym Challenger to support Rose during the rebellion, and she garnered a reputation as a fierce and unbreakable fighter, similar to Gloria's reputation for the GNLF. After the government's defeat, she retreated south to the Crown Tundra to join Oleana in her insurgency. Following Oleana's capture Vega went missing, however alleged sightings of her have continued to this day.
The names of the Gym Challengers are taken from the Bulbapedia article of the "Gym Challenger" trainer class from Sword and Shield. Also, I must thank the Fantasy Name Generators website for producing the last names of the trainers and the website "Wheel Decide" to determine the fate of each trainer.