The gate is like a capacitor and will hold a charge (and voltage) unless you allow for a drain-off path. A resistor to source will do the job. You'll need the lowest value you can tolerate (to reduce turn-off time) that will still allow the gate to come fully on when the transistor comes on.

How about if I stuck a diode in there, would that work?

 

That actually blocks the very current path that's required. Replace the diode with a resistor, probably 10K or so would be fine.