The documentation for “Discourse on Ghost” that I’m finding is here: Home | Discourse on Ghost. There’s also a related topic by the code’s author on the Ghost Discourse forum: ✨ Introducing Discourse on Ghost - Reimagining the integration! - Share & showcase - Ghost Forum.
If I’m understanding things correctly, Discourse on Ghost is primarily (?) intended to be used with self-hosted Ghost installations with Discourse on Ghost installed on the same server as the Ghost installation is run from. There’s also an option to use JSON Web Token authentication. It seems that method would allow you to install the Discourse on Ghost application on a server that you control, while having both Ghost and Discourse hosted on paid plans.
My understanding is that this means that there isn’t currently a non-technical way of getting this to work. For your case, you could have your Ghost blog hosted on the Ghost platform and your Discourse site hosted by Discourse, but you would need to setup a server on a VPS to run the Discourse on Ghost application: GitHub - vikaspotluri123/discourse-on-ghost: Add Ghost-based SSO to Discourse.
That seems like an interesting problem, but maybe it’s more technical than what you are wanting to get into.
It would be great if the Ghost platform supported DiscourseConnect out of the box, in a similar way to how platforms like Memberful support DiscourseConnect.