How to download a VM image from GCP?
I don't know of a way to get the actual GCE VM images.

But it's possible to export the images to GCS (as a tar.gz archive):


  If you need to move your Compute Engine boot disk data outside of your
  Compute Engine project, you can export a boot disk image to Cloud
  Storage as a tar.gz file. 
  
  You can export a custom image as backup or for sharing by exporting
  the image to Cloud Storage. This method is ideal for sharing
  individual images with other projects that do not have access to your
  images.


You can then download this tar.gz file and, I guess, create a VM image from it to run on your local machine. I imagine the process would contain steps similar  to those detailed in Migrating VMs to Compute Engine and Importing Boot Disk Images to Compute Engine, only seen from your VirtualBox perspective instead of the GCE one. 

These guides are making it easy to transfer VM loads to GCE. I guess they don't have incentives to do the same for transfers from GCE, which probably explains why there is no guide for it ;)