Gambling winnings and prizes are taxable income even if there were no tax withholdings when you were paid or received your prize.
Go to this section in Credit Karma Tax: Gambling Income
The IRS requires that you report your gambling winnings as income. If a payer of gambling winnings withholds income tax when they pay you, you should receive a Form W-2G, Certain Gambling Winnings showing the amount you won and the amount withheld. Not all amounts or wagers are subject to withholding at the time of payment so you may have winnings to report even if you did not receive a Form W-2G.
Deducting gambling losses
If you itemize your deductions, you can deduct any gambling losses up to your total winnings but you’ll need clear and accurate records of these losses. In Credit Karma Tax, you can claim your gambling losses under Miscellaneous Deductions - Schedule A.
Source: irs.gov