You can add a custom server connection using Forge. However Forge is build to set up a server for you on one of the third parties. However GCE is doing the same thing for you.
It should be possible to add a custom server using the "custom VPS" option. You need to make sure that you have a user called root. I'm not sure if GCE will provide you with such a user. If that isn't the case it's currently not possible