how to allocate w2-g to my schedule c

There is no way to link the income reported on your W-G form to your schedule C. What you can do is report the gambling winnings like you normally would, in the Less Common Income section of TurboTax, then put in a negative adjustment to income to back the income out. Then, enter your winnings in the self-employed income section of TurboTax as "General Income" not reported on a 1099 form.

You can make that adjusting entry in TurboTax as follows:

1. From the Federal menu in TurboTax find Wages and Income 

2. Find Less Common Income

3. Choose Miscellaneous Income, 1099-A, 1099-C

4. Choose Other Reportable Income

5. Enter a description of the adjustment and the adjustment as a negative number