The mattermost server doesn't send a 504 error. This appears to be coming from your proxy. Are you using nginx? Can you look in those logs for any corresponding errors?
10.5.5 504 Gateway Timeout
The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server specified by the URI (e.g. HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed to access in attempting to complete the request.
Note: Note to implementors: some deployed proxies are known to
return 400 or 500 when DNS lookups time out.