“When I start the game, I get a “Gepard::GT Code: 300″ error which mentions it cannot validate the license on the License Server.”

This is caused by a firewall blocking either the Gepard Shield IP addresses or, most likely, the ports. For starters, make sure port 6700 is unblocked in your firewall. Often this error happens on those who are on a work or university network. So also make sure it’s unblocked in the network in general. Additionally, make sure IP addresses 185.61.138.117 and 139.99.40.25 are not blocked. Gepard Shield uses these for its license server.