GST ANX-I is now available as out of the box in the latest cumulative update of Dynamics NAV 2016.
You can view the cumulative updates here.
Before you install the latest update, get your license refreshed. You can contact your partner and get the license refreshed.
Let's take a quick look into the GST ANX-I, below screen shot,
Click on the GST ANX-I, you should get the request option to select File Format.
So in here lets select Purchase register and execute the report, as the report finishes it creates an excel file.
In this way you can export the required type of supply data.