1. Go to transaction code SMGW. Choose Goto > Expert Functions > Logging. From here, you can define the File Name for the log and specify which Log Events to be monitored. Choose the Toggle Criteria and activate the trace logging. The logging will be keep generating even if the gateway is restarted but will be stop if the instance is shutdown or restart.
2. For permanent setting, the parameter gw/logging need to be maintained in transaction code RZ10.
gw/logging= ACTION=SPX LOGFILE=gw_log_$(SAPSYSTEMNAME)_$(SAPLOCALHOST)-%y&m%d SWITCHTF=day
3. You can get the trace file in work directory.
