If you are driving the gate from a MCU port pin, you dont need a pull-down resistor. However, the IRF520 requires +10V on its gate to fully turn it on. Your MCU will supply a maximum of 5V (its Vdd voltage). To do this properly, you will likely need to replace the IRF520 with a modern "logic gate" NFET that turns on fully with less than 5V on its gate.
If all you need to do is to illuminate a 20mA indicator LED, then you do not need an NFET at all. What is the real load you are trying to switch with the NFET?