Gauss Rifles have negative OEN values, how does that work?

Treat it the same as any weapon. If someone is in no armour, they take DMG 3. If they are in PV2 armour they take DMG3 - (PV2 - (-4)) = 0 damage.

The coding is designed so that if you have any armour you'll just get hit by bruising damage, but no wound. If however you're wearing nothing, then expect a broken limb.

Stormers, Necanthropes and Shaktar are considered to have PV1 when attacked by a Gauss Rifle (ie: it does no damage against them except through bonus damage from a high hit score of 20+)