Hi Suman,
The Push based publish and subscribe scenario works in such a way that whenever an event occurs, it announces(Publishes) its occurrence and the subscribers who have subscribed to that event will get notification regarding the event occurrence.
In your case, whenever there is a SO created, it will publish its occurrence(this triggering event can be anything like your scheduled job can also publish the event occurrence). The Gateway will receive this and will send the notification to the server(3rd party) provided that the server has subscribed for the event.
If I explain it technically, there will be 3 systems(or 2 systems based on your GW deployment approach) i.e. Backend ECC, Netweaver Gateway and your third party server. Firstly you need to create a GW service with SO as an entityset (which can be subscribed). Now your 3rd party server needs to subscribe for this entity (this will be done by triggering a POST call with some subscription payload). At this point of time these two systems are ready in terms of sending the notification to the server.
Now we will focus on ECC which will push the data to the GW system. There is one report program which is used to push the notifications to the GW system. You can call this report inside your scheduled job FM which will push the required SO information against the SO entityset. As soon as this event information is pushed from ECC to GW, GW will push this information to the 3rd party server as XML notification. Your 3rd party system should be able to receive this XML notification and parse it to get the req. information.
You can get the step by step information regarding this in the links shared in previous reply.
Regards,
Ekansh