One of the biggest mysteries in The 100 season 7 is what happened to Gaia, who was seemingly kidnapped toward the start of the season and hasn't been seen or heard from since. Since it's The 100's final season, the writers and producers have chosen to tackle as many storylines as possible, including people from up to five different factions, and everything is finally starting to come together.
Sheidheda's reign over the remaining Grounders, Eligius prisoners, and Sanctum natives has been strenuous at best, and the Bardoans mission to get their hands on the Key (the Flame) and begin the Last War has put them at odds with Clarke and her friends (but not all of them, since Bellamy has sided with his Shepherd, Bill). And amidst all of this, one thing many of the characters fail to realize is that Gaia is missing.
Gaia was with Clarke and their friends when they went through the Anomaly's portal in The 100 season 7, episode 4, "Hesperides", but she chose to remain behind in order to protect Madi. Once everyone had left, Gaia was attacked by someone dressed as a Disciple, who used the suit to power down Sanctum's Anomaly stone, possibly for good. But before the portal closed, the Gaia and the Disciple fell through it. So who took her and where did she go? One of the better theories circling around online is that Gaia and the unknown Disciple are stuck in the bridge between worlds.
Right before Gaia attacked the Disciple, the unknown person began the Anomaly stone's shutdown sequence, and they both fell into it the second before the portal closed. Given the timing of it all, it's reasonable they're both trapped between the various connected worlds. It's unlikely they were transported to Skyring/Penance, Nakara, Bardo, or even Earth. They were already on Sanctum, so that's out, and the only other option is Etherea. Since Bellamy was there for several months - if not a year - he would've seen Gaia at some point. That is, of course, if he was really there. Taking all of that into account, the logical solution is that Gaia is trapped, and the Disciple, whoever he or she is, is trapped there with her.
What's interesting is that the series hasn't addressed this particular question at all, despite Gaia being a part of so many characters' lives. Now that Clarke is back on Sanctum, she'll likely be informed by Murphy and Indra about what happened while she was gone, at which she'll surely ask about Madi and Gaia. Since the mystery surrounding Gaia's disappearance hasn't affected the storyline too much, it seems her arc isn't as important to the overarching story as viewers thought. But then again, since this is The 100, Gaia could return at the last second with integral information about the Flame and the Last War. She was the Flamekeeper, after all.