To enter the W-2G For your Gambling winnings--Go to Federal>Wages & Income>Less Common Income>Gambling Winnings
You can enter your winnings, and then keep clicking through the interview to enter gambling losses.
Gambling losses are an itemized deduction; you can only get a deduction if the combination of all of your other itemized deductions exceeds your standard deduction. Losses do not offset winnings dollar for dollar.