The only way for this to work, they would have already had a number of tracks completed & a number of
tracks in different stages completion, that way they would have had a good head start at the beginning. I could see getting the first few tracks on schedule, then some delays between the remaining tracks, kinda have that with the Vision cars already.
At the time of GT5, there was some speculation that not all the tracks where released in time for the
game, though never confirmed, those tracks that where still in the works would be released at a later time.
I assumed that some of these tracks where the tracks being refereed to, going to be for DLC.
7 months into release is a little concerning, excluding the two tracks included with the Senna content,
I would have expected the first few by now.
It would seem if a statement is made “
at least one additional track per month will be downloadable.” which is a bold statement, that they would have already had some completed and ready to go by now.
Here is one news feed, there is a earlier one I can't find.
https://www.gtplanet.net/gran-turismo-6-tracks-dlc-monthly/