I assume this is because you want to install some Beta plugin?
1) Go to SiteAdmin->Plugins.
2) Click the "Change URL" button.
3) Type http://beta.gpsgate.com
in the "New URL" field below, then hit Enter.
Now you can update either an existing plugin to Beta version or install a new one from the "Get More Plugins" tab.
It is recommended to test Beta plugins on a non business-critical server first (depending on what advice you got from GpsGate support).