Your gain is the difference between what you paid for the stock and what you sold it for, it doesn't matter that the sale was forced. If you got the stock through your employment, your cost is what you paid taxes on as part of your taxable income.
Capital gains are taxed as regular income if held one year or less, and as long term capital gains if held more than one year. Long term capital gains tax rate is 15% for most taxpayers and 20% for high income taxpayers, you can see the brackets here.
https://www.nerdwallet.com/article/taxes/capital-gains-tax-rates
Your state will likely tax your gains as regular income, state tax rates are 3%-13% depending on the state and your income.
If you made an estimated payment now of 10% to your state and 20% to the IRS, you would likely be more than covered, and might get a partial refund.
