Gunnarsson scored 3:51 into overtime for a 3-2 win against the Boston Bruins in Game 2 at TD Garden on Wednesday that evened the best-of-7 series. Game 3 is at St. Louis on Saturday.
St. Louis was 0-13 in the Cup Final, including Game 1 of this series (4-2 on Monday).
"I don't score too many, but nothing comes even close," said Gunnarsson, whose goal was his first in 57 Stanley Cup Playoff games. "It's a pretty good feeling right now."
WATCH: [Blues vs. Bruins Game 2 highlights | Complete series coverage]
Gunnarsson told Blues coach Craig Berube during a quick exchange in the bathroom between the third period and overtime that he felt he needed one more chance after his slap shot hit the right post with 1:57 remaining in the third.
During a delayed penalty, the Blues kept possession and Gunnarsson scored on a slap shot from inside the blue line.
"First game, we weren't happy; today we came out flying and I think we played a great game for 60-plus minutes," the defenseman said. "That's just the team we are. We come back like that. We never give up. For me to put it in there and get the winner, it's a great feeling."
Tuukka Rask made 34 saves for the Bruins, who had their eight-game winning streak end. It was their first loss since April 30.
"Probably wasn't our best game today," Rask said. "I think, especially in the playoffs, the better team always wins. They were definitely the better team."