Four goals in the first 32 minutes helped Valencia to a comprehensive victory which moves them up to second in Group A.
The hosts stormed out of the blocks and peppered their opponents' goal with shots early on.
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Valencia doubled their lead shortly afterwards in slightly fortunate circumstances. Cartabia raced forward from midfield and unleashed a 30-yard drive which deflected off a defender and into the goal, with the goalkeeper wrong-footed.
Cartabia got his second of the game on the half-hour mark, firing the ball towards goal from the right and seeing squirm under the goalkeeper and into the net.
The Spanish side were awarded another free kick moments later and this time Canales stepped up, whipping the ball over the wall and into the bottom corner.
St Gallen did manage a consolation goal in response when Stephane Nater scored from close range after a flick-on at the front post, but it did not make much difference in the end.