Ratesetter loan payments are taken via Direct Debit.
giffgaff cannot remove a direct debit from your account and Ratesetter can’t discuss the loan unless you can prove you’re the one who took out the loan, which you say you are not.
Please contact your bank via the number on the back of your debit card and ask for the fraud department. They will be able to advise and assist you.
Don’t remove the direct debit yourself, especially without contacting your bank first.