I did some testing yesterday. The light gauss is effective against the Harasser and light mechs, where two rounds will knock out between 60-75% of almost any armour section. Front, side, turret, torso, leg, or arm.
In medium mechs you already begin to lose effectiveness, where the Shadowcat and Bushwacker don't take significant damage across major armour sections. The damage is alright if you're hitting them near max range, or they're not paying attention to you, but you'll probably lose in a slugging match. Limbs and shoulder mounts should be your main targets, although they're harder to hit.
Heavy mechs barely take 10-15% damage against a leg hit even. A torso shot is even more pitiful, although shoulder mounts still take a significant amount of damage.
I didn't even bother with assault mechs.
(Some other testing: ERPPCs butt rape anything less than a heavy, two gauss rifles + extra ammo make for a good amount of butthurt, and the 2 MRM40 catapult is some sort of Atlas raping god with a nearby APC for refills.)