If you are talking one of those Big Square 12V batteries I say yes, you are not testing a large load.
Test leads ( you will need 4)
10 ohm 1 watt resister
10 ohm 5 watt resister
As for ground... your thinking earth ground like for an AC system in your house. The term "ground" and "negative" are used interchangeably. The negative post on the battery is the ground, if it is attached to the metal of the gauge cluster, the cluster is "grounded" and any wire you attach to the metal will also be grounded.
I'm starting to really understand how this stuff works (THANKS!), but still don't understand how to mimic the resistance at the sending unit to check the gauges. This is resistance in the flow to ground, correct?