Gus will use Graye’s mirror earring as a Chekhov's Gun to foil Belos' plans in the Season 2 Finale.
Back in "Hollow Mind", Eda and King rightfully point out to Luz that if she has to break 50 (actually centuries) years of totalitarian, social-darwinism mindset and to expose Belos as a delusional, genocidal fraud, she must find and show people concrete proof. Since Graye's mirror earring has the power to amplify Illusion magic by showing people's memories, Gus will use it to give Luz and the people of the Boiling Isles concrete proof about Belos' true colors, leading to his downfall.- Confirmed. Gus does use the trinket in "King's Tide", but it only manages to stall Belos briefly.
