You can deduct gambling losses on Schedule A as a Miscellaneous deduction that is not subject to the 2% limitation. (Schedule A Line 28.) You cannot deduct more losses than you won. Here's the bad news. If your Total Itemized Deductions, on Schedule A Line 29, does not exceed your standard deduction, then the losses will be of no tax benefit.
In previous response, I forgot to tell you that if your winnings are on W-2G, then TurboTax will automatically prompt you to enter your losses after you enter the data for W-2G. TurboTax will put the loss onto Schedule A. As before, if your total deductions are less than your standard deduction, then you will not see any benefit for the loss.