Yes, even if no federal income tax was withheld, your gambling winnings are still subject to federal income tax and need to be reported on your return.
If you are itemizing your deductions, you may be able to enter gambling losses to help offset some of the winnings. After you enter your gambling winnings in the Income section (search for W2-G in the search box), continue in the interview and the program will ask you about any gambling losses. You will only be able to take a deduction for the amount up to your winnings and your deduction will be an itemized deduction, so it will not be a direct offset of the income.
