That's not much to go on and there is a lot to consider. Forgive the questions, but there is a method to my madness.
Was this recently working normally? Any notable events at the time it started mis-behaving?
Does the engine turn over easily/quickly when it won't start?
Does the pump stop (reaches pressure) before you cycle the ignition switch? Does everything run normally after it is successfully started? Does it start easily after sitting for short periods of time? Have you done a fuel pressure test to ensure the pump is working properly?
Given it's fuel pump and gauge related, it's worth checking the wiring/connectors to the pump.
A FULL reading is an indication of low resistance, implicating the sender or crossed wires. (Reading EMPTY would be an indication of high resistance/break in the circuit.)
And of course, low battery capacity is known for contributing to starting issues on these machines, so a load test might prove useful. A failing battery sitting for a few days might be discharging enough to make starting difficult. Battery fatigue generally shows up as the climate gets colder.
If the kill switch is two-pole like the Zs, it's worth testing as well since they are known for developing high resistance (poor contact) on the EFI pole.