To do this you can use the following steps:
- Insert knob block from simulink library browser in your model
- Double-click on knob block, block parameters dailog will open
- Select the block whose parameter you want to control using knob, for eg. Gain block in your case
- A list containing all the possible tunable parameters of that particular block will appear, for eg. Gain for Gain block
- Select the paramter you want to tune (Gain here) and then click apply
- Now to change the Gain value using knob, first click on the knob, blue outline will come, showing knob is in active mode
- Change the position of the knob to change the Gain.
Refer to the model enclosed.