The win in front of 4,758 fans at MTS Centre put the defending world champions right where they want to be - in Tuesday's gold-medal game.
The only thing the Americans have to do now is wait to find out who they'll play, which will be the winner of Monday night's game between Canada and Finland.
Canada and the U.S. have met in all nine world championship finals. The American's only gold was a 1-0 shootout win in 2005.
There was no world championship last year because of the Olympic Games in Italy, where Canada won gold over Sweden. The U.S. lost to Sweden in a semifinal shootout and settled for bronze with a victory over Finland.
The Canadians defeated the Americans 5-4 in a shootout on Saturday with Jessie Vetter in net for the U.S. It's expected the veteran Gunn will get the nod for Tuesday's game.
Sunday's game marked the ninth world championship game between the Americans and Finns. The U.S. has won every game except in 1997, when the teams skated to a tie.
The Americans peppered Finland goalie Noora Raty with 37 shots.
American forward Natalie Darwitz scored the game's first goal at 9:53 of the first period after sending the puck behind Raty from the side of the net.
American captain Krissy Wendell made it 2-0 with just under two minutes left in the period. With an assist on Darwitz's goal, Wendell leads the team in scoring with four goals and 11 points.
Canadian veteran Hayley Wickenheiser is the tournament's leading scorer with 12 points, including six goals.
Finland had a two-man advantage for 1:02 early in the second period, but couldn't even get a shot on Gunn.
Defenceman Molly Engstrom scored the only second-period goal and also recorded an assist on Wendell's goal.
Forward Kristin King skated across the front of the net and flipped a backhand past Raty at 3:31 of the third period for the 4-0 lead.