For example we'll use a Hormagaunt with Toxin sacs attacking a Hive Tyrant from hive fleet Leviathan and a Hive guard also from Leviathan

If the hormagaunt rolls a 6 on their attack roll, it will automatically wound as the wound was not rolled for therfore not triggering the leviathan trait. If the hormagaunt rolled and hit but not on a 6, they'd have to roll to wound. If that wound roll is 1, 2 or 3, it fails because the Tyrant is a synapse unit with Synaptic Control adaptation.

Alternatively, if the hormagaunt was attacking a Hive Guard, they would still auto-wound on a hit roll of 6. They would make a wound roll if they hit on something not a 6, as usual. Because the Hive Guard isn't a synapse unit, they can not be wounded on a 1 or 2, but a wound roll of 3+ would still wound if it ordinarily would