pull the sending unit and see if it has gas in it.
did you get the correct unit for your voltage system? 6v and 12v need i believe 30 and 60 ohms but don't quote me on this. just know they are different.
check grounds as shipwright says, especially in my 6v system, grounding is 98% of all problems.
the instrument should already been held to the dash with some nuts that provide ground, so i wouldn't think you'd need extra grounding there but it never hurts.

Overland Blue, 1953 GMC 1/2T, 100-22, 228/1bbl, 4sp/4.10 6v, shortbed/fender side/5-window.