Gideon is an old schoolmate of Hamid and Bertie from Cambridge University.
He now attends Prague University, where he was the assistant to Einstein. He's engaged to Liliana Beekos, Hamid's ex-girlfriend.
Physical appearance
Tall and well-built, looking like the "captain of the rowing club."
History
According to Hamid's story in episode 95, Hamid met Gideon while attending a rich boarding school in England when Hamid was 15. Gideon was a bully who "took active pleasure in being mean to people", although he was good at covering his tracks. Hamid became his best friend, and a bully, to avoid being a target as a brown halfling boy who didn't speak English well. Saira apparently never liked Gideon and disapproved of Hamid being friends with him.
Hamid and Gideon both went to Cambridge, where Gideon felt he could do whatever he wanted and still enjoyed making other suffer. He also met Bertie at this point, and Gideon ended up owing Bertie money. At Cambridge Hamid tried to avoid Gideon at first, but he eventually fell back into old habits and they started spending more time together again.
Gideon orchestrated a prank on a professor he hated, involving "an alchemist’s thing" that "was supposed to just pop, and sizzle, and stink"; he had Hamid write the formula. However, Hamid wrote the formula down wrong, Gideon doubled the quantities, and the classroom where they set the prank contained students. Several of the students died, along with the professor, who, by Liliana's comment in episode 83, seems to have been the dean of the university.
At some point in the months between Hamid being expelled from Cambridge and visiting Prague with the LOLOMG, Gideon moved to Prague. He entered into a relationship with and became engaged to Liliana, lying to her that Hamid led him astray and told him to switch the formula. He also became Einstein's assistant.
Season 2
The LOLOMG meet Gideon while he's working as Einstein's assistant in Prague. Although Hamid casts Invisibility on himself multiple times to hide from Gideon, they both clearly recognize each other, though they try to hide it. Gideon expresses interest in finding out where Hamid is, though won't say why. Sasha and Grizzop conclude that Hamid and Gideon are exes. Gideon also tries to avoid Bertie, who (because of Mr Ceiling) does not remember the debt that Gideon owes him.
Gideon later announces the presence of the Cult of Mars at the university in episode 80, although he does not escape with the professors; it's unclear what happened to him subsequently.