When you had the sender out and were checking travel, did you try it connected to the gage? Alligator clip a local ground and run it thru the swing and watch the gage.
This article is the easiest IMO on the swing setup top and bottom of tank.
https://www.hotrodhotline.com/instal...uge-and-sender
I almost bought one of these gage/sender calibrators, but found my issue was the gage wouldn't return to zero (mind of its own) and purchased a Speedhut programmable gage instead.
Classic Instruments Store / SN34
https://www.speedwaymotors.com/Class...ce,230181.html
http://www.classicinstruments.com/us...n%20020116.pdf
Edit
I just re-read you 1st post.
-"when I ground the sensor wire, the gauge reads full." When you ground it your giving the gage Zero Ohms. AKA= low OHMs when full, High OHMs when empty.
Later you wrote.
- "On the bench, the sensor gives 25 ohms at empty and 125 ohms at full(float up). IF so your sender is 180 out from what your gage needs for an input.
It is possible to flip the float (loosen the set screw and run the rod thru from the other side) So that down is up per se. Take a look at the article I 1st posted above from hotrodonline.