What you describe is not a gift in any way, form or fashion. It's a loan. You don't pay taxes on borrowed money, because it's not "your" money. However, if you pay interest on that loan, then the recipient of that interest has to report the interest they receive from you, as taxable income.
You may (or may not) also find that a reputable lender will not approve your mortgage either, as you having to pay back the loan for the down payment is a liability that counts against you from the mortgage lender's perspective. Generally, that down payment is expected to be "your" money. When the lender is pulling your credit, they're going to want to know where you got that down payment from. If the down payment is truly a gift, then they will most likely want legally enforceable paperwork to indicate that, and that you are under no obligation to pay it back. If the "gift" is over 14K to any one individual in a tax year, then the giver of the gift has to file a gift tax return with the IRS - but you already know that I take it.
