Gambling losses can be deducted up to the amount won, but only as an itemized deduction.

Go through the income on the Gambling winnings again as after you are done entering it, there should be a screen open that has you enter your losses, and it will only allow amount up to winnings.

If this does not open, then go to Deductions and Credits section down to bottom to Other deductions and credits and to subheading "Other Deductible Expenses" and it will get enter here.  This first option should work however. 

Gambling Losses are reported on Form 1040 Schedule A as a Miscellaneous itemized deduction.  Gambling losses are not a one-for-one reduction in winnings.  Your total itemized deductions reported on Form 1040 Schedule A must be greater than the standard deduction for your filing status to have any tax benefit.