The input to ' Gate Driver' block can be both a Physical signal input, Electrical input ports.
Current the ' Gate Driver' block that you are using has only a physical signal as input. You need to convert it to electrical input port to connect with ' Controlled PWM voltage block'. To do so right click the block in your block diagram and then select the appropriate option from the context menu, under Simscape > Block choices.
For more information please have a look at the following link:
https://www.mathworks.com/help/physmod/elec/ref/gatedriver.html
