How can I track my direct selling revenue if I published my app via TGC publishing service?

The details of this service have not yet been released. I doubt you will get a full answer until these details have been decided and released.

I imagine it will be fairly well documented.

We're still working on the submission and back-end system, but the general idea is that you will log into your TGC Account as normal and a new option will be available where you can see at a glance all your apps, all the sales that app has made, and the royalties you are due to receive. We already do this for third party developers who sell through the TGC site so it's something we are quite familiar with.