That will be for refilling the DEF (Diesel Exhaust Fluid) when the diesel models come out...Yes, one HUGE drawback (and another expense added into the vehicles cost of ownership) to an Obama-EPA diesel vehicle...
They just pre-designed the panels and parts so it's cheaper to build the diesel Gladiators when they do start making them. With the diesel Gladiators come out, there will be another hole, fill-neck, and blue plastic cap right there.