A Gravity belt is a magical item used to levitate its wearer off the ground at various levels of altitudes.
During a reconnaissance operation for a bank heist, Penny, Julia, and the rest of the Physical Kids were discussing the procedure in which they'd be able to secure the gold stored in one of the underground vaults.
Penny argued that a bank, of all places, was the most likely to be warded against Travellers, to which Margo agreed, stating that the vault's floor was one big Anti-Traveler Ward, clarifying that once he got in and touched the ward, he couldn't get out, and would lose consciousness in five minutes afterwards due to lack of oxygen in the vault.
However, Margo followed up by stating Penny wouldn't be trapped as long as he didn't touch the vault floor. But, with Penny's hands still healing after being severed by The Beast, his magic wasn't reliable, aside from his Traveling.
Margo then offered a solution in the form of the Gravity Belt, a specialty of Physical Kids in particular, that would enable him to remain off the ground without having to cast magic, as the magic to keep him airborne would instead be cast upon the belt, allowing him to Travel in and our with no issue.