I think I missed this message. In simulation, it is more direct to calculate charge with a current source at the gate, as Q = I * time. With constant current source, by controlling simulation run time, we control how much charge injected. We can use a voltage source there, but eventually, we still have to add a series resistor and also probe the gate current and calculate the charge. Using voltage source just make the calculation more complex and indirect. But both can achieve the same purpose.
FET is voltage-controlled as the channel is controlled by gate voltage, but when driving a FET, it is more like current drive. To increase the gate voltage, we have to charge up the FET’s parasitic capacitance (e.g., Cgs, Cds) to reach the desired gate voltage. To drive a FET to quickly turn on/off, the charging and discharging current can be several amps at peak, which can be a quite heavy depends on switching frequency. At MHz application, it can even heat up the FET just because of this gate current.