If there is a firewall somewhere, either deployed by your hosting company or embedded in Centos, then you need to open the relevant ports. You probably already did this to get your game server to work in the first place. If not, then there probably aren't any ports to open.
If I were you, I'd try to start the server from the command line instead of from GC, since that provides you with lots more info of what's going on. If the server runs nicely from the command line, you can try to query it on its query port using hlsw or a similar tool. If you get a response, then that is not the problem.
If you do not get a query response, but the query port is open, it is rather likely that the server is unable to start properly when metamod is enabled. Perhaps the server fires up, then stops with an error message or freezes? In that case, the GC will not restart it since the process is running, but the GC server will restart it after a few minutes since it is unable to communicate with it. You will see this easily if you start from the command line. Try to compare with/without metamod running.
I assume you use the exact same command-line as GC uses, so that all settings and config files are the same?
After startting manually, are you able to the server on the query-port from a computer on a different network? If not, then you need to figure out why that fails, as that is the reason why GC assumes the server doesn't run (query failure) and orders a restart.
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