As always it’s hard to troubleshoot by comment, but a few things to try:

Outside of your Vue component object, set a local variable instead to access google.maps:

<script>
  const GMaps = window.google.maps
  export default {
    ...
    mounted() { ... GMaps ...}
  }
</script>

I recall an issue I had a long time ago where Vue was looking for a Vue variable called window and causing that error.

If that doesn’t work, try and use google.maps instead of window.google.maps.

But I suspect neither will work and so if you can replicate the issue in a code pen or git, I’d be happy to help troubleshoot myself.

As I say at the top of this, this is just a tutorial and x5-gmaps is a stable package I actively support and develop.