The gauge grounds through the water temp sending unit. That's a yellow green wire.

There is a combined ground with the fuel gauge, a white and black wire. Grounded over at the left front pillar.

The hot lead coming in to all the gauges is yellow.

To measure the sending unit, the resistance should be 146.6 at 140F, and 24.3 at 239F.

To check the gauge, measure resistance between the following points on the dashboard gauge cluster.
IG-TU 135
TU-E 138
IG-E 273