Gauge is an indication of level in the tank based on the float position, DTE is an average based on how the car is being driven at that time, it’s constantly updating, so driving hard will drop it while cruising for a bit in sixth gear at a steady speed will see it jump up, if after driving for five min at a steady pace it does not increase, there may be an issue, but it’s not likely.
About that ~2 gal mark is when it blanks out. Lawyers don't want the responsibility when someone runs out of gas while its still reading a few miles remaining.
Dealership time
Manufacturers have to account for different circumstances and leave a bit extra in the tank - for example, a car parked on an incline in San Francisco needs enough fuel in case the pickup is on the high side of the tank. If the display shows 20 miles remaining, then park your car on a hill, a problem occurs when you can’t warm up the car and you drain the tank despite being told you had 20 miles left.
I had exactly the same experience in my 24 Performance Z!
