I guess because no work has been done to get the two apps talking to each other yet... here's how to get it working;
1. If you haven't got MC12 installed you need to install it AND then G-Force. This won't affect your MC13 installation (unless you install it in the same folder as MC13, but don't do that).
2. Run REGEDIT from the command line and go to the HKEY_LOCAL_MACHINE\SOFTWARE\JRiver\Media Center 12\Plugins\Display\G-Force key.
3. Right-click on the G-Force key in the tree view and then Export to a filename of your choosing.
4. Edit the resulting .reg file in notepad and where it says HKEY_LOCAL_MACHINE\SOFTWARE\JRiver\Media Center 12\Plugins\Display\G-Force change to HKEY_LOCAL_MACHINE\SOFTWARE\JRiver\Media Center 13\Plugins\Display\G-Force.
5. Save the .reg file and then double-click on it in explorer to import the registry settings.
6. Copy C:\Program Files\J River\Media Center 12\Visualizations\G-Force_JMC.dll to C:\Program Files\J River\Media Center 13\Visualizations\G-Force_JMC.dll.
That should do it, good luck.