Basically, Gertrude sold her heart to Monday at a fairy market in order to learn magic more quickly and get her greatest desire (to be loved for who she is without compromise). And Monday stated the deal would take away her fears and help her grow without boundaries.
@Legendary_Sidekick and I have played her since then as an IDGAF teenager. But you could certainly interpret or tweak it however you like.
Here are
the slides for our B&B. Gertrude sheet has the updated mechanics and her contract with Monday is in there.