Gregoire and Secretary of State Sam Reed certified the election on Wednesday afternoon, as they were joined by couples who plan to wed and community activists who worked on the campaign supporting gay marriage.
The law doesn't take effect until Thursday, when gay and lesbian couples can start picking up their wedding certificates and licenses at county auditors' offices. King County, the state's largest and home to Seattle, and Thurston County, home to the state capital of Olympia, will open the earliest, at 12:01 a.m. Thursday, to start issuing marriage licenses.
Because the state has a three-day waiting period, the earliest that weddings can take place is Sunday. Same-sex couples who previously were married in another state that allows gay marriage, like Massachusetts, will not have to get remarried in Washington state. Their marriages will be valid here as soon as the law takes effect.
"This is a very important and historic day in the great state of Washington," Gregoire said before signing the measure that officially certified the election results. "For many years now we've said one more step, one more step. And this is our last step for marriage equality in the state of Washington."
Last month, Washington, Maine and Maryland became the first states to pass same-sex marriage by popular vote. They joined six other states New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and the District of Columbia that had already enacted laws or issued court rulings permitting same-sex marriage.
Referendum 74 in Washington state had asked voters to either approve or reject the state law legalizing same-sex marriage that legislators passed earlier this year. That law was signed by Gregoire in February but was put on hold pending the outcome of the election. Nearly 54 percent of voters approved the measure.
The law doesn't require religious organizations or churches to perform marriages, and it doesn't subject churches to penalties if they don't marry gay or lesbian couples.
Heather Kawamoto and Kay Lancaster of Tacoma attended the signing event Wednesday afternoon with their 9-year-old daughter, Kayleigh Kawamoto.
Kawamoto and Lancaster have been together more than 14 years, and domestic partners since 2007, and both said they can't wait to finally pick up their marriage license as soon as the Pierce County auditor's office opens at 6:30 a.m. Thursday. They will marry in a small ceremony on Sunday in Tacoma.
"It's something we've hoped for and dreamed of," Lancaster said. "I didn't dare hope that it would be this soon, and we're just thrilled that it is."
Lancaster and Kawamoto said that the reality of their impending marriage sunk in in the past few days, as they were writing their vows.
"We never knew we'd be able to say those things to each other," Lancaster said, starting to cry, as Kayleigh quickly handed her a tissue.
Hundreds of people lined up in downtown Seattle Wednesday night to become among the first in the state to get a marriage license as a same-sex couple.
Brenda Bauer and Celia Castle were among those who showed up to make history.
"Life is challenging enough without having to fight to just have your relationship recognized, have your family recognized," Bauer said.
The couple has been in a relationship for 24 years, and both are ready to say "I do."
"When you say you're married, everybody completely understands what kind of commitment that is," Bauer said.
The county put up tents to accommodate the crowing crowd, and officials say they expect nearly 18 hours of sign up, which is unprecedented.
"People who have been waiting all these years to have their rights recognized should have not to wait one minute longer," said King County Executive Dow Constantine.
In addition to private ceremonies that will start taking place across Washington state this weekend, Seattle City Hall will open for several hours on Sunday, and several local judges are donating their time to marry couples. Aaron Pickus, a spokesman for Mayor Mike McGinn, said that more than 140 couples have registered to get married at City Hall, and weddings will begin at 10 a.m.
Washington state has had a domestic partnership law in place since 2007. The initial law granted couples about two dozen rights, including hospital visitation and inheritance rights when there is no will. It was expanded a year later, and then again in 2009, when lawmakers completed the package with the so-called "everything but marriage" law that was ultimately upheld by voters later that year.
This year, lawmakers passed the law allowing gay marriage, and Gregoire signed it in February. Opponents gathered enough signatures for a referendum, putting the law on hold before it could take effect.
There are nearly 10,000 domestic partnership registrations with the secretary of state's office. Most same-sex domestic partnerships that aren't ended prior to June 30, 2014, automatically become marriages, unless one of the partners is 62 or older.
That provision was included in the state's first domestic partnership law of 2007 to help heterosexual seniors who don't remarry out of fear they could lose certain pension or Social Security benefits.
Marcy Kulland and Terry Virgona, both 59 and from Tacoma, said they plan to get married on Sept. 28, 2013 to celebrate their 22nd anniversary.
"I'm just ecstatic. Now we're legitimized," Kulland said. "It's just absolutely wonderful."
However, she that while the state law is a great step forward, as long as federal law continues to deny federal recognition of same-sex marriages, there's more to be done.
"This completes us, it doesn't complete our work," Kulland said.