[87Skier]

Welcome to the forum! I see that you posted in the 69-72 Blazer section, but you have a 77. It's usually best to post in the 73-87 section, or one of the subsections covering the 73-91 Blazers and Suburbans.

Anyways...

I happened to have a gauge cluster that is identical to yours.
Looking from the back of the gauge:

Left post - Fuel signal
Right post - 12v signal power
Bottom - ground

You can trace the circuit board and should be able to see that the left post does not go to any lights. It goes from the gauge directly to the connector.

Alternatively, you can test this setup by supplying 12 volts to the right post and grounding the bottom. The gauge should go to about 3 o'clock (way passed the full mark). Shorting the left post to ground will cause it to go to completely empty.