"Game Error G99" is an error message you can get in H1Z1
There is an issue connecting to the login server. The login server may be down for maintenance. Please try again a little later. Be sure to check the forums for announcements on planned server shutdowns and estimated downtimes. If there has been no notification of a server shutdown, validate your game files from the LaunchPad and make sure that H1Z1.exe, awesomiumprocess.exe and launchpad.exe are all set as exceptions in your firewall/antivirus program(s). The login server may still be shut down without notification, but running through these steps will ensure that the problem is not on your end.
Overload on login servers
Wait a few minutes and try logging in again