Wow: a dollar for the actual mosfet and 4x that for the board....
What board is that?
Um ... Didn't the board already supply a Gate grounding resistor?
What board is this? Where is/ was the link to a board?
The gate grounding resistor is to keep the FET switched off when the Arduino is reset and its pins are effectively inputs and may float. Since this problem belongs to the Arduino rather than the FET, the resistor should be across the Arduino pin to ground. If it is on the FET gate, after the series resistor, then it tends to form a voltage divider and reduce the voltage turning on the gate. That may be a minor effect, but it makes sense to avoid even minor impediments to performance.
Also, anyone got any idea what is the gate-source diode doing in RFP30N06LE?
To protect against ESD (Electrostatic discharge).