"Would the contact pad on the back of the gauge that would normally be grounded via the body, play a part in testing?"
If the actual fuel gauge itself is removed from the gauge cluster housing/body. The answer is Yes, the contact pad/plate is the ground for the fuel gauge. The fuel gauge pad/plate transfers the fuel gauge ground to the gauge cluster housing/body. When installing the fuel gauge into the gauge cluster housing there is a insulating pad that prevents the hot fuel gauge terminals from touching the gauge cluster housing. "The gauge housing was not getting a good ground due to corrosion on the back of the dash."
That could be a possibility but uncommon.
Testing the fuel gauge when installed in the gauge cluster housing is preferred. The Ground wire then can be attached to the mounting plate..
