Gaelets are individuals who will lend money to those without sufficient collateral or familial connections to obtain loans through proper guild lenders, at an exchange rate more than double their legal counterparts. They have a reputation for being fearsome and ruthless, liable to go after you directly if you cannot repay them in time, and borrowing from one is usually a last resort.
Unlike legal moneylenders, gaelets will not necessarily be Cealdish.
In the chronicle
Throughout Kvothe's time at the University, he was forced to borrow his tuition money at an exorbitant interest rate from a gaelet in Imre named Devi. She requested a drop of his blood in exchange for the loans, which she claimed that she would sell if he were late on his payments. If late on payments, she would also accept information or favors in lieu of money. This was seen as a good deal, compared to "someone ready to beat twelve distinct colors of hell out of you if you were a day late."
There were mentions of a man named Heffron and "his boys", who are also gaelets in Imre. Sleat is implied to work as a gaelet, among other criminal activities. A gaelet character named Ketler is present in the play "The Ghost and the Goosegirl."