Output current capability of a waveform generator
According to the datasheet you linked to (see page 4,) the output impedance is 50 ohms:

The MOSFET will act as though there were a 50 ohm resistor between the signal generator output and the gate.  That is, you use the gate capacitance and 50 ohms to calculate how fast the MOSFET can switch.
This question and its answers give tips on calculating the switching time.