Beginner Question- DC Motor Speed

Hi all- We are beginners to Simulink. In a training we attended, we were told to use the Gain block to control the motor speed. We are just trying to do a basic program to make a robot (Vex) go forward. Anytime we change the Gain to anything higher than 1, the robot becomes a little unresponsive to the controller. For example, it'll keep driving for a few rotations even after we have let off on the joystick. Any ideas? I feel like the Gain block isn't the right choice for this, but I have no clue what to use in its place. TIA!