Assuming you paid taxes on it in a prior year, you can deduct it from your taxable income in the tax year you paid it back.
Under the Deductions & Credits tab scroll down to the Other Deductions & Credits section and under that section elect to start/update Other Deductible Expenses.
For Generation skipping taxes click NO.
For repayment of less common expenses, click NO.
For "Do you have any of these other deductions?" note that one of them is for repayments over $3K. So click YES and you should be able to take it from there.