I am trying to test android deep link urls through adb to launch my app

You don’t need to specify full path to your activity, but if you want to test whether you react properly to URI in your app just specify app package:

adb shell am start -a android.intent.action.VIEW -d "example://gizmos" com.myapp

Also there is bug in command you provided – there should be example://gizmos not example:gizmos