From what I could see gerbv currently expects that each argument that is not an option is a valid gerber/drill file. The problem is that if you just configure to open .gvp with gerbv it fails to do so because your OS (I'm on Windows btw.) doesn't know about the needed --project="..." option. You currently have to manually modify the command line used to open your project file, which may or may not be easy depending on your OS.

Another problem with projects: The file init.scm that seems to be required for opening a project isn't packaged with the builds.