You may open the server to the internet for testing but will not recommend to keep it open all the time. You will be hacked.
When you created the virtual machine disk (VHD) on VirtualBox did you had the network set as Bridged Adapter?
Did you try the login using the gcloud sdk? I also can not SSH to the server but I can login using the SDK.
Also you can try the steps in [How-to] Install Freepbx distro (with commercial modules) on Google (cloud) Compute Engine using this iso image https://downloads.freepbxdistro.org/ISO/SNG7-FPBX-64bit-1805-2.iso
One thing you may try is Google support. If your account is less than a year old you get free email support.