It looks like Gauss cannons can do critical damage even if the armor is not stripped to 0, this fits with the lore. Gauss in the lore have the unique ability of penetrating through armor and doing some damage to internal components even if the armor is still there.
I'm pretty sure this is not the case. I don't think there are any weapons in MWO that can crit through armor. I believe in tabletop there are a couple of weapons or special ammo types that can do it but nothing that has made its way to MWO.
