Backup the database using the utility gdmmsconfig.exe in program files\G Data Management Server\ and import it into the new management server using the same tool.
The clients will not know the new server but you can try installing the G Data client remotely again from the console. You will get a message that the client is already installed but it will change a key in the registry so that the client will attach to the new server:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\G DATA\AVKClient\server
You can also change this value manually in the registry on the client and enter the new server name or ip address.