The term saturation is not used with MOSFETs, that's for BJT transistors.
In case of a MOSFET, as you will see from the datasheet, the higher the voltage at the gate (until it reaches the limit, typically around 20V), the lower the on resistance.
So when you said "gate fully saturated" I was guessing you're driving it with a higher voltage, like the 12V of the circuit you're switching. But that's not the case, it's just 5V that you apply to the gate.