An important point is that we might not actually have 6 turns of delay here. Why? Well, the later that we generate a Great Person in Marble, the more dillution our Scientists will have on the Great Person probability of becoming a Great Prophet. So, preferably, Marble City will be one of our LAST Great People to be spawned. Since it will already have a good number of Great Prophet Points, we will most likely delay hiring Specialists here for quite some time.
We can't delay GP generation here that long. It has to produce either our 2nd or 3rd GP(preferably third). This is because if we generated a GS from there the best use for it is to lightbulb optics, and if we got a prophet and needed to self tech it then we would need to start researching it before the 4th GP was generated.