The donation to the Initiative 594 campaign was given Friday but not made public until Monday, when it posted on the state's Public Disclosure Commission website.
The large donation comes on the heels of fellow Microsoft co-founder Paul Allen's $500,000 donation earlier this month. Also last week, venture capitalist Nick Hanauer donated an additional $1 million, bring his total donation to the campaign to nearly $1.4 million.
Initiative 594 would require background checks for all gun sales and transfers in Washington state, including at gun shows and for private sales. Under the measure, some exemptions would exist, including gifts within a family and antiques.