Correct, that loan is not income. You can code it as a transfer. If you can link that loan to Tiller, then great. My guess would be that you can. If not, create a liability on Balance History to get the liability on your balance sheet (Balances). You can call the liability a loan payable.
When you loan money to another, you have an asset called loan receivable. You can code the money going out to the borrower as a transfer. Create an asset called loan receivable to get the asset on your balance sheet (Balances).
When you receive a payment, split the transaction into two pieces, interest income (code as income) and loan repayment (code as transfer). If the payment includes principal, then make a manual entry on Balance History to reflect the new reduced loan receivable balance.
To keep all the numbers straight and to be sure everyone is on the same page, an amortization schedule is recommended. See this link for a template to create this schedule yourself. Loan Amortization Schedule and Calculator
Also, I would recommend a written loan agreement if for no other reason than to have support for the existence of the loan for tax purposes in case you ever encountered the unfortunate situation where you had to write off the loan as worthless and uncollectible.