Gouge was recently-ish changed to allow gouging from the off-hand, not just the primary hand.

Expected: Gouge with a non-dagger mainwield and dagger off-hand will use the material and damage type of the dagger to calculate damage.

Actual: Gouge seems to use the mainwield weapon's material and damage type.

When gouging a juju zombie with a straight razor (no vuln) mainwielded, this was the max damage I was doing:

You lunge at the juju zombie and gouge it with a straight razor!
You drive the dagger deep into the juju zombie's arm.
Your gouge DISMEMBERS the juju zombie!

When gouging a juju zombie with a silver longsword with an iron horseshoe pommel (double vuln) and the razor in the off-hand, this was the minimum damage I was doing:

You lunge at the juju zombie and gouge it with a straight razor!
You drive the dagger into the juju zombie's chest.
Your gouge *** DEMOLISHES *** the juju zombie!
The juju zombie is covered with bleeding wounds.