VirtualBox Extension on Google Cloud Compute Engine

I was trying to install the VirtualBox 4.3.20 Oracle VM VirtualBox Extension Pack (found here) on my Google Cloud Compute VM Instance after setting up the X11 forwarding so that I could use the GUI for virtualbox.

I downloaded the extensions file, then opened the GUI, chose "File" --> "Preferences" --> Extensions and then the button to include extensions (the blue box with a down arrow). From there, I directed it to the file I'd downloaded and it wanted a password to confirm that it was okay to install the extension file... but I have no SSH password because that's not how the Cloud Compute Instance works...

So, I logged in from my machine's terminal again, but this time where it has my machine name (eg: "machine-name"), I put "root@machine-name" instead. Then, I went into the GUI, clicked on the "Computer" option, then "home", then the file in question and it installed beautifully.

Google Cloud Compute X11 Forwarding

I'm setting up a virtualbox instance on Google's Cloud Compute Engine. When I try to ssh in on my terminal with the given command they provide plus the following argument after "ssh"

--ssh-flag=-X

I got the error: "X11 forwarding request failed on channel 0." 

First, I updated /etc/ssh/ssh_config by running sudo [editor] /etc/ssh/ssh_config, uncommenting the following lines and putting "yes" where there was a "no."

   ForwardX11 yes

   ForwardX11Trusted yes

Then, I updated the /etc/ssh/sshd_config file, so that after the other X11 related lines it also said the following:

X11UseLocalhost no

Then, I rebooted the machine (sudo reboot) and redid the ssh command with --ssh-flag=-Y to do X11 forwarding. It worked great!