On the documentation it says:
On Android, the map is rendered beneath the entire webview, and uses this component to manage its positioning during scrolling events. This means that as the developer, you must ensure that the webview is transparent all the way through the layers to the very bottom. In a typically Ionic application, that means setting transparency on elements such as IonContent and the root HTML tag to ensure that it can be seen. If you can't see your map on Android, this should be the first thing you check.
So you need to alter your css so that it shows on Android, something like this should get the job done:

ion-content {
  --background: transparent;
}

body {
  background: transparent;
}

However, doing these css changes will cause a lot of unwanted style changes and issues. Moreover, based on my experience trying @capacitor/google-maps, I would highly recommend using @angular/google-maps instead, as it is more developed, maintained by google and the capacitor plugin still has a long way to go.
You can check this post on ionic's forum as I had the exact same issue.

UPDATE:

You can check @angular/google-maps here , also you can check below a sample code:
HTML

<google-map [center]="{lat: 20, lng:20}" [options]="options" height="140px" width="auto">

    <map-marker [position]="{lat: 20, lng: 20}" [options]="markerOptions"></map-marker>

</google-map>

ts:

markerOptions: google.maps.MarkerOptions = {
    draggable: false,
  };

  options: google.maps.MapOptions = {
    zoom: 13,
    disableDefaultUI: true,
    draggable: false,
    clickableIcons: false, 
  };

You can also see Google's documentation to get a list of all the options for their map