MOSFET drives high voltage and high current load with PWM
You already included all you need to look for.

The gate output voltage of the MIC5018 is specified as min. 11.4V @ V[supply]=4.5V, typically above 15V @ 5V.

You want a FET with V[th] preferredly lower than (11.4V-5V)=6.4V (e.g. logic-level MOSFET), or at least lower than 15V-5V=10V, current capability 20A at given V[gs] of 6.4...10V, min. 48V V[ds] rating.

Besides the requirement to be able to carry 20A at the available V[gs], you want to look for a reasonably low R[ds] at that gate voltage to reduce losses and heat in the FET.