You are seeing triggered alerts in the event log for the "Gateway is not alive" metric despite the gateway being online.
By default, the gateway will check in with the management server every 5 seconds. The management server, in turn, is configured to check every 30 seconds, that there have been 24 reports in the past 2 minutes.
Check and ensure that the time on both the gateway and management server is in sync. You can check the current time using the "date" command from CLI, or look for the time under Management, Audit Log, in the upper right-hand corner to see the time on the management server.
If the time is out of sync, you can specify/update the NTP server from: System Configuration, Gateways, by editing the specific gateway. To force an NTP update, you can use the command "ntpdate" from the CLI.