It looked like Gushue, from St. John's, N.L., was sure of the prize after a big four in the fifth end broke open a close game.
But McEwen came back in six with a four of his own on a spectacular triple-raise takeout, after Gushue hogged his attempt at a final guard, that sent them to seven tied 6-6.
With his 8-6 cushion from seven, and a big hit that cleared McEwen from the house, Gushue ran McEwen out of rocks and they shook in eight.
They blanked the first and McEwen stole one in the second. Gushue had a deuce in the third end to go ahead 2-1 and McEwen had to settle for a single in four to tie it up going into five.
Gushue took home $23,500, 12 Rogers Cup points and McEwen had to settle for $14,500 and nine
McEwen has been hard to beat this season with four World Curling Tour wins and an overall record of 33-2 before Sunday.
McEwen demolished Olympic gold-medal winner Brad Jacobs 7-2 in five ends Saturday night to get to the final while Gushue needed an extra end to edge Toronto's John Epping 6-5.
McEwen's only loss during the week was to Calgary's John Morris, who took over Kevin Koe's Brier-winning rink after Koe struck out on his own with a new team. Gushue's only loss was to Manitoba's Jeff Stoughton.