Arduino Voltage Divider Wrong Voltage
Vgs(th) is the voltage at which the MOSFET turns on, not the maximum Gate-Source voltage the part can tolerate.

Maximum Vgs is given in the "Absolute Maximum Ratings" table of the datasheet and is +/-12V so you don't need to "protect" the gate from the voltage level of your Arduino.

As to why your voltage divider doesn't work as expected: Have you verified your component values and your source voltage?