If you want to keep a record of the money you lend or owe someone, you can use Assets and Liabilities.
When you lend someone money – create an asset. Assets are normally used for objects of value that you possess, like a car or a house. You can't use an asset as a payment account (Income/Expense transactions), but you can edit its value or sell/purchase it. Use the type of transaction "Asset Purchase" when you lend money from an existing payment account and "Asset Sale" when you get the money back.
Similarly, when you borrow money from somebody, you can create a liability. When you pay it back, you can register the transaction as a "Discharge of Liability" to pay from an existing payment account or "Liability Acquisition" when you add borrowed money to one of your accounts.
You can create assets or liabilities without influencing your payment accounts. To do that, go to the Balance tab and tap "Edit" > "Add". You will be offered to create an asset or a liability.
You can use the transaction "Money Transfer" to move money between your payment accounts and assets/liabilities.