How are these gateways managed precisely?
Are you using an external security management server and if so what version.
For R77.20 in particular, it's important to know what hardware as there are different OS variants which have different steps.
Note that generally, from these versions, there either is no mechanism available to export logs via syslog or the mechanism to do so is lacking.
As those versions are not supported any longer (except on some SMB appliances where R77.20.87 is the latest release available), your best bet is to upgrade to a supported release.
For self-managed SMB appliances in particular, the ability to directly export via syslog is only supported in the 1500 Series running R80.x code and isn't available in models still running R77.20 code.
In regular R80.x gateways, Log Exporter can be used to send logs via syslog.