Fuel gauge

[Rig7]

I don't understand electric wiring, I am a dummy at it. That being said I could use some help with my fuel gauge problem. I have a 1969 GMC C1500 truck, the gas tank is still mounted behind the seat. I have a 1/4 tank of gas in it. My gauge is reading past full at the 3 o'clock position. What I did was hooked up my test light to the ground checked it with some of the fuses to make sure it was working correctly, it was. So I touched It to the wire and sending unit the wire that goes back to the fuse box, when I did that the gauge moved back to the 1/4 tank mark. Know isn't that grounding that positive wire to the sending unit? I just don't know what to do at this point. Need help please.