Where do I enter a form w-2g, certain gambling winnings? If I select add another w-2, it has required fields that are not on the w-2g.

Are you in the spot for "regular" W-2's?  That is not where you want to enter a W-2G

Gambling

To enter the W-2G  or other documents 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.

https://www.irs.gov/help/ita/how-do-i-claim-my-gambling-winnings-and-or-losses

Gambling winnings are taxable income.  Losses are an itemized deduction.  If you do not have enough itemized deductions to exceed your standard deduction, your losses will have no effect.

https://blog.turbotax.intuit.com/income-and-investments/how-are-gambling-winnings-taxed-8891/

https://ttlc.intuit.com/questions/1900352-can-i-deduct-my-gambling-losses

2020 Standard Deduction Amounts

Single $12,400   (+ $1650 65 or older)

Married Filing Separate  $12,400   (+ $1300 if 65 or older)

Married Filing Jointly $24,800   (+ $1300 for each spouse 65 or older)

Head of Household $18,650  (+ $1650 for 65 or older)

Gambling losses can only be deducted up to the amount of the gambling winnings. Gambling winnings are reported on Form 1040 Schedule 1 on Line 8 as Other Income. 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.

To enter, edit or delete Gambling Winnings and Losses -

Enter w2g in the Search box in the upper right of the program screen. Click on Jump to w2g