The sync in is retriggered by a zero crossing of another wave, normally they would use a saw, dont tell anyone but you can actually use anything. but squares can give a bit of a "nasty" transient.
the sync output on a sync master osc is nothing more than a sawtooth wave.
thats not to confuse "sync" with an LFO, which does use the gate. it makes it start at the same point in the wave cycle when a note is pressed. if you want your oscs to do that you can use an LFO as an audio osc. and it will always start at the same place when a note is pressed. (and you can even adjust where with the phase control)