resolve class name

Hi everybody, I try to call Gate::inspect() with a variable class:

$this->model = 'App\Models\' . ucfirst($model_action[0]) . '::class';
// holds correct classname as string: "App\Models\City::class"
$response = Gate::inspect($this->action, App\Models\City::class);
// hardcoded Model returns allowed()=true as expected
$response = Gate::inspect($this->action, $this->model);
// variable Model returns allowed()=false

I know the problem is that $this->model is a string and the CityPolicy is never called by Gate in my variable model code. Could someone help me out how to "cast" the string value so that Gate::inspect will accept it as the Model?

Thx Marco