Should be marked:

S - Sender (should be a single Tan wire)
I - Power/ignition (should be gray wire, goes to (or from) the voltmeter)
G - Ground (should be a black wire, goes to (or from) the voltmeter)

There are the light wires as well (black and purple).

Looking at the gauge, going clockwise:

Top (12 o'clock) is tan (sender)
Right (3 o'clock) is gray
Bottom (6 o'clock) is light (violet and black, violet got angry with me for calling her purple above!)
Left (9 o'clock) is black.

Both black and gray will usually have multiple wires, and go directly to the volt meter in most trucks.

Peter