Would a "fuel leak" at the gate be a reportable incident?
There are a couple of places in the regualtions that require FAA or NTSB notification of certain events:


14 CFR 121.703 Service difficulty reports
14 CFR 121.705 Mechanical interruption summary report
49 CFR 830.5 Immediate notification (to the NTSB)


The closest thing to what you described would be the following in 121.703(a):


  (11) A fuel or fuel-dumping system that affects fuel flow or causes hazardous leakage during flight;


Note that it says during flight though, so this wouldn't apply and they wouldn't be required to report it.