I understand this may be a very late reply, but I have linked steps to troubleshoot 404 errors on the gateway in this topic Unable to access my API
From the looks of things, db_app_conf_options seems to be missing from your gateway config file. I also notice a bunch of audit issue with your API definition:
listen_portof your API definition is set 80 and not the same as the gateway’s 8080 (i.e. setting listen_port=0 will use the default value from the gateway).domainof your API definition is set to listen on a custom domain and not the host of the gateway