You will need to amend the 2018 tax return to add the W-2G and losses. 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 21 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.
An amended federal tax return, Form 1040X, can only be printed and mailed, it cannot be e-filed. The IRS will take up to 4 months or longer to process the amended tax return. If taxes are owed after completing the amended return, you send a check with the amended return when it is mailed. If the amended return has a tax refund, then the IRS will send you a refund check in the mail for the refund, the refund cannot be direct deposited.
The tax return should not be amended until the tax refund has been received from the IRS.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed