Gauge is working, but I second the spitting. Have to let it "burp" every 15 seconds or so. Filler gun diameter and hole in filler cap fitting are very close. So, if there IS a vent under, it's not working. If it's designed to vent (in and out) through the cap, I need to think of something.
Ironically, that's how cars used to work until the EPA decided that the 12 gas molecules escaping into the atmosphere were a threat to civilization and they mandated an air tight filler cap (leave it off and see how fast you get a check engine light) and a complex system of vent hoses, and feeds back to the engine.
Apparently they don't know about our fuel tanks......SHHHHH!