The Swiss side were humiliated at the Mestalla, barely getting out of their own half, especially as Valencia dominated the whole of the opening 45 minutes.
Fede Cartabia scored his first goals for Los Che as he bagged a brace, while Paco Alcacer and Ricardo Costa were also on target before the break.
Only brilliant St Gallen keeper Daniel Lopar kept the score down, with Sergio Canales beating him for a fifth time in the second half before St Gallen somehow scored through Stephane Nater.
Valencia now sit second in the group on six points, just one behind leaders Swansea, with St Gallen in third and horribly beaten.
The writing was on the wall for them from the get-go as Canales drew a save from Lopar with just 43 seconds gone, with the goalkeeper working again three minutes later as Juan Bernat cracked an effort in.
Canales then took aim with no joy again but it was no surprise that the scoring started with 12 minutes gone as Alcacer finished after an already overworked Lopar could only parry Pablo Piatti's shot.
Mario Mutsch registered St Gallen's first shot in anger but it offered nothing more than momentary respite as Cartabia whipped in a fine second from the edge of the area with 20 minutes on the clock.
It was difficult not to feel for Lopar as Alcacer and Dani Parejo both stung his hands, but despite his excellence he was picking the ball out of the net for a third time in the 30th minute.
Cartabia was the scorer again, showing a killer streak for a second time as he ran on to Joao Pereira's pass and finished from the edge of the box.
Ricardo Costa made it four three minutes later as he nodded in after a Victor Ruiz flick and had Piatti shown greater poise from six yards, it would have been five. The same applied to Parejo and Piatti again just before the whistle was blown.
Piatti ensured Valencia started the second half on the front foot too, although the finishing was not as sharp, with Canales and Alcacer off the mark.
Lopar kept the score at four when he repelled Juan Bernat, with Piatti wide on the rebound, but the inevitable fifth was scored with 20 minutes remaining.
Canales got it, whipping in a 25-yard free-kick that added further balance to the scoreline, but that was quickly cancelled out as St Gallen miraculously got their act together and scored through a 74th-minute Nater tap-in.
That would be that, though, and Valencia coasted through to the end.