Drain switching is somewhat complex, as one need to be assured the bias conditions are stable before applying and signal to the gate. I am assuming you are familiar with stability circles and the like and have performed the required analysis for your wanted operating conditions. Be aware that the steady state large signal S-parameter may differ significantly from your pulsed large signal S-parameters (not an easy measurement, btw) which may invalidate your initial stability analysis, but if that's all you have it's a reasonable starting point. At a push, even the small signal S-parameters are better than nothing. GaN devices suffer more than GaAs from internal heating effects, due to their smaller geometries and higher energy densities - there's less chip backside area to conduct the heat away. The canon of knowledge on GaN is significantly smaller than for GaAs, although that may have changed since I last looked.
Obviously, when drain switching, there is a certain amount of time required for the bias to stabilize - this is dependent on the device and duty ratio and power.
If your application allows, using class B or C operation is the simplest way to go which avoids the need for drain switching, but you will generate more harmonics, which is a problem unless you have a tuned load. Also remember that filters generally reflect out of band power, which might upset your device.
Always ensure that your device is protected from operating into an open circuit - one way is to use a isolator on the output - many power devices have been destroyed this way.
Do not expect to be able to simulate the behaviour of these devices fully - you will have to experiment - and you will lose a few devices along the way!