The round traveling through the air at high velocity will still make a sound when it exceeds the speed of sound. The rules are probably just referring to the lack of explosive sound from the propellant.
The problem, however, with suppressing a gauss rifle is two fold: first, suppressors work by forcing the explosion inside the gun to be dispersed through baffles to reduce the sound of the explosion. So this wouldn't work on a gauss rifle. In fact, suppressors really only work well (and at that they really just - except in some specific weapons - just lower the sound enough that it doesn't sound so much like a gun anymore, but they are not entirely silent except for those aforementioned special instances) if the weapon is single shot, or "sealed" - a revolver isn't a good choice for suppressing a weapon.
Second, suppressors use subsonic rounds to help lower the crack of the round itself as it travels through the air. Otherwise you get the same sonic boom effect a plane will cause at mach speed, just smaller. So that kind of limits the punch of a gauss rifle considerably. In spite of what you see in the movies rifles are not usually the weapon of choice for suppressing - SMG's or purpose-built carbines are because they use a smaller, more easily quieted round in close quarters. After all, if you fire a high powered rifle far enough away from the target, he'll be dead well before he hears the round firing. So not only is suppressing a rifle difficult, it takes away a lot for the weapon's utility as a long range weapon.
IMTU, gauss weapons make a distinctive snap! when fired; a little quieter than a regular slug-thrower but only in that they don't go BOOM and have muzzle flash.