Based on your description it's not a gift, it's a loan, since you are expected to pay it back.
You do not report the loan on your tax return. It has no effect on your taxes. There is nothing you can claim as a deduction.
The "small amount" additional that you pay back is income to your father-in-law. He has to report it as interest income on his tax return for the year that you pay it. There is a minimum interest rate, called the Applicable Federal Rate, that he is assumed to have earned, even if you do not actually pay that much. If you pay less than the minimum rate he has to report interest income as if you paid the minimum rate. See IRS Tax Rules for Imputed Interest.