The punishment will drop the Spaniard from eighth to 11th on the race grid.
The incident happened in the final minutes of the Q1 segment of the session and forced Gasly to abandon his lap and fail to progress to Q2.
The French driver was incensed when he came upon Sainz, driving slowly at the approach to the chicane at the Circuit Gilles Villeneve, and had to swerve into a run-off area to avoid a collision.
"It's completely unacceptable to be driving the way Carlos did," said Gasly. "It's just as simple as that.
"I was coming at 300 and he's sitting in the last chicane just focusing on his own lap -- but you're not alone on the racetrack
"First of all, I could not even complete the lap, which would have put us in the top six quite easily and second, it was extremely dangerous and it was just unnecessary. So I'm just absolutely gutted."
Race stewards investigated after the session and duly gave Sainz the three-place penalty.
