The Gifted has wrapped its first season and even though the X-Men didn't appear, they loomed large over the series. Set in an alternate timeline from the X-Men films, The Gifted carved their own indelible version of a world where mutants exist and are not only hated and feared as socio-political pariahs, but are often hunted and incarcerated as terrorists. However, the series made it abundantly clear from the start that even though they are now missing, the X-Men remain an integral part of this world and its history.
The details of which particular mutants comprised the X-Men aren't clear, but it's safe to say they were assembled by Professor Charles Xavier, and that Warpath, the older brother of John Proudstar AKA Thunderbird, was part of their ranks. This family connection is why Thunderbird was chosen to lead the Mutant Underground network. Lorna Dane AKA Polaris was recruited into the Mutant Underground by a mutant attorney named Evangeline Whedon because of who her father was: Magneto, the mutant master of magnetism but also the dearest friend of Professor X.
THE HELLFIRE CLUB
Whether Magneto was ever counted as one of the X-Men isn't yet known, but what is known is Magneto was a King of the Hellfire Club. Similar to how they were depicted in X-Men: First Class, the Hellfire Club's heyday was in the 1960s, which is presumably when they crossed paths and were dismantled by Charles Xavier's first class of X-Men. However, the Hellfire Club regrouped in the decades since and has become resurgent by The Gifted's era, with the Frost Triplets (also known as the Cuckoos and possibly related to the Hellfire Club's White Queen Emma Frost) working explicitly to bring the club back to the heights of its prominence.
Other villains who were part of the Hellfire Club are Andreas and Andrea Von Strucker, collectively known as Fenris. Two of the most wanted and destructive mutant terrorists who ever lived, the great grandparents of The Gifted's teenage heroes Lauren and Andy Strucker were last seen on the run from the X-Men. It's not yet known whether Fenris is dead or remain at large.
The incident that occurred in Dallas on July 15, 2013 changed everything for mutants in the United States. A mutant civil rights protest turned violent and tragic, with thousands dead, humans and mutants alike. One of the resultant surges of energy from mutant powers gone amok caused the death of Grace Turner, the 7 year old daughter of Jace Turner. Jace was driven to join the anti-mutant government organization Sentinel Services to avenge his daughter, becoming of the most determined enemies of the Mutant Underground. Laws were quickly enacted restricting the civil rights of mutants, with violent use of mutant powers automatically branding mutants as terrorists.
Within a year after the event that became forever known as 7/15, the X-Men disappeared, as did their enemy collectives the Brotherhood of Mutants (which is believed to be led by Magneto) and the Mutant Liberation Front. What happened to the X-Men isn't known but they have been gone long enough that many mutants consider the X-Men urban legends - or perhaps the idea of the X-Men as folk heroes is kept alive to give despondent mutants hope. Lauren and Andy Strucker, however, 'played X-Men' when they were younger.
THE MUTANT UNDERGROUND
Before the X-Men disappeared, they created the Mutant Underground, a network based on the Underground Railroad of the 19th century. After placing Thunderbird and Polaris in charge of the cell based in Atlanta, they went on to recruit other mutants to the network. One of their most important recruit was Marcos Diaz AKA Eclipse, who signed on and quickly became one of the leaders of the Mutant Underground and Polaris' lover.
Ultimately, the X-Men left the Mutant Underground with relatively limited resources before they vanished, so the mutants' struggle is a difficult one without the safety of a school, a Blackbird stealth fighter jet, and all the perks the students of Professor X enjoyed.
The Gifted season 1 is streaming on Hulu.