$gatebias

To determine the bias added to the gate contact. It should be written in one row with three numbers which represent the start of the bias voltage the end of the bias voltage and the step size of the voltage in sequence. For example:

$gatebias 
-0.3 0.3 0.1 

This means that the bias of the gate contact will starts at 0V and increases 0.1V every step until the gate bias reached 0.4V.
i.e. -0.3 -0.2 -0.1 0 0.1 0.2 0.3

The $gatebias setting in GUI interface is here
Press Contact Setting and set up the bias of the gate contact!

$drainbias, $sourcebias