How to determine the value for the gate resistor (Rg) of a MOSFET in low side configuration?
In general the comparator has to supply the gate charge to turn the MOSFET on and off. With a gate charge in the 100nC range, 1mA will allow switching within 1ms or so. 

You can compare the 400mA maximum load current and the switching time to the SOA (Safe Operating Area) of the MOSFET you have in mind. The worst-case stress on the transistor is when it is turning on (or prematurely off) with a few hundred mA flowing and most of the supply voltage across the MOSFET. 

And, BTW, don't depend on Vgs(th), depend on the guaranteed Rds(on) with your minimum supply voltage. For example, this one (SI4448) which is guaranteed to have an Rds(on) of less than 2.5m\$\Omega\$ with drive >= 2.5V. 

The comparator specs are guaranteed with 4mA output and capable of about 50mA drive, so you could arbitrarily pick a resistor in the 100 ohm to 1K range. If you check the the SOA on the SI4448, you'll see that it's not limiting for your situation. 

You need a diode across the motor to deal with the motor inductance, or else you'll  probably avalanche the MOSFET with the voltage spike when the MOSFET switches off.

Probably not an issue in your case, but also note that if the motor was externally driven in reverse it will produce voltage which could cause issues.