I received a letter from irs showing my gambling winnings that I did not claim in 2019. Am I able to go into my 2019 and claim my winnings and losses?

Yes, if you can itemize on your 2019 tax return, then you may be able to claim gambling losses. You can read more about it here and here. 

Thank you! So I can still add in my gambling winnings and losses AFTER they sent me the letter of what I owe? Because if I add in what I lost, I will be paying a lot less then the irs “proposal letter” 

Yes, you can add you gambling losses even if the IRS sent you a proposed changes letter. See Can I deduct my gambling losses?

You may need to amend your 2019 income tax return and send it in with the IRS letter.