Assuming you have your app running when you want the model to update the Gauge, you could achieve this a couple of ways. In both cases, you'll need a handle to the running app, which is returned as a left hand side argument when you first launch the app, i.e.
1) Update the gauge by accessing it directly in your Simulink callback. Assuming the name you gave the gauge in App Designer's Component Browser was "GainGauge", you'd use the syntax:
2) Alternatively, create a public function in your app that takes care of updating the gauge or any other updating logic you need, and call that function from your Simulink callback. Assuming the public function you create is named "myAppUpdateRoutine", the syntax you'd use to call it is:
Hope this helps!