Current consumption of PWM controlled MOSFET
I'm trying to design an application in which a DC/DC converter with
  limited output current capabilities drives the gate of one or more
  transistors in PWM mode


There may be some confusion here. You drive the gate of a MOSFET typically with a pulses of varying mark-space ratio. That is a standard way of achieving PWM control. Whether ultimately the pulses are derived from a DC to DC converter is a detail that is largely irrelevant to working out how much power is wasted driving the MOSFET gate.

The gate-source loss comes about because you charge a capacitor to some voltage then you discharge it. 

To calculate that loss, use the energy equation for a capacitor, work = \$CV^2/2\$. So, if the capacitance is (say) 5 nF and you charge it to 10 volts, then you are putting in an energy of 250 nJ. When you discharge it that energy is lost forever so, if that happens at a switching (PWM) frequency of 100 kHz, the power lost in driving the gate is 25 mW.

If you have 6 similar MOSFETs in your motor controller then the total power wasted in driving them at 100 kHz is 150 mW.

Regards the maximum current injected into the gate to raise it to 10 volts, you use the formula I = C dv/dt where dv/dt is 10 volts divided by the rise time of the gate drive. If you want very fast switching (not always desireable) than you need more current.

For the example of 100 kHz PWM, the switching period is 10 us and if you said that no more than 5% of the time was spent charging up the gate then dt = 500 ns.

I = 5 nF * 10V/500 ns = 100 mA.

If you wanted the switching 5 times faster then current would need to be 500 mA.