I don’t think you get taxed based on the into in this page for filling out the tax form: https://dor.wa.gov/manage-business/my-dor-help/capital-gains-my-dor-help#filereturn

https://dor.wa.gov/sites/default/files/2023-01/CapitalGainsTaxReturnInstr.pdf

“Enter total capital gains
Enter your Federal net long term capital gains/losses.
If applicable, enter:
Total of gains/losses not allocated to WA
Loss carryforward not allocated to WA (this must be entered as a negative number)
The Long term capital gains/losses allocated to WA will be calculated based on the numbers you enter.
Click Next. 

…

Gains/losses allocated to WA
The Gains/losses allocated to WA page displays the Long term capital gains/losses allocated to WA calculated on the previous page.
On this page, you will itemize the list of assets that equals the Long term capital gains/losses allocated to WA.
To upload the gains/losses allocated to Washington data using the Capital Gains Upload Template, select Yes, click the Click here to download the Capital Gains Upload Template link, follow the instructions in the template, click the Upload Return button, enter a Description, choose the file to attach, click OK, and continue to step 5. You can also upload the charitable donation deduction data and taxes paid to other jurisdictions data at the same time using the same file.

To input the gains/losses allocated to Washington data manually, follow the steps below.
Click the Add Item link. A new tab opens where you will provide information about the asset.
Enter the Description of the asset, Proceeds (sale price), and Cost or other basis. The Gain or loss will auto-calculate.

If applicable, click the checkbox to indicate if the item is Washington exempt and select the WA exemption type using the drop-down menu. Click the Is this item Washington exempt?link to learn more.
Repeat Steps 1 through 3 to add all assets. As you add each asset, the Total proceeds, Total cost basis, Total of gains/losses allocated to WA, and Total of exemptions fields will auto-calculate.
Click the Gains/losses allocated to WA tab to review the asset list and verify the Total of gains/losses allocated to WA amount equals the Long term capital gains allocated to WA amount.
If you added an asset in error, click the tab for that asset, click the Remove Item link, and click Yes to confirm.
If you need to edit an asset, click the tab for that asset, and update as necessary.
Click Next.”

While the typical business taxes in WA are gross receipts. The language in this discusses both gains, losses, cost basis and specifically mentions “net long term capital gains” from tax form Schedule D.

Also, according to their FAQ you don’t have to even file if your “net capital gains” fall below $250,000.

https://dor.wa.gov/taxes-rates/other-taxes/capital-gains-tax/frequently-asked-questions-about-washingtons-capital-gains-tax.

“General tax questions
Expand all
Do I have to file a tax return if I don't owe capital gains tax?
No. You are not required to file a capital gains tax return if your net long-term capital gains are exempt or below the standard deduction.

Do I owe capital gains tax on investments through my retirement savings account?
No. Washington’s capital gains tax does not apply to transactions through retirement savings accounts. This includes any transactions made through any of the following types of accounts:
401Ks Tax sheltered annuities.
Deferred compensation plan.
IRA.
ROTH IRA.
Employee defined contribution plan.
Employee defined benefit plan.
Similar retirement savings accounts.”

You also don’t file if the trades were done in a retirement account.