Hi,
What specific version of CallManager are you currently running? Since you say the gateway is not registering with CCM, I assume this is an MGCP gateway, correct? If no changes were made to the configuration of the gateway, have you tried resetting the gateway within CCM or resetting the MGCP process on the gateway from the IOS CLI (no mgcp, then MGCP)? Post the config and output of 'debug mgcp packets' captured when resetting the GW.
Regards.