I think it's a variety of factors all hitting at once for Gatsby. Phantom's absence is certainly one of them. I think Eva and Jeremy for sure helped sell out the Paper Mill run in like, a day, and it felt more focused on the two of them out of town, and what a limited special experience that was. Now, it seems like a big plus that they have two great stars, but the show itself has found its footing, it's not a "one month only!" type deal, and I don't think anyone would blink twice if you put two other equally largeish theater names in there to replace them.
Without them, though, I'm not sure it would have reached the success (or at least sales consistency), it has today. Were they the it factor before? Yes, I think combined with the existing IP, that Eva and Jeremy got the show to this solid place. However, I don't think they're as crucial to this show's ongoing success as they were in getting it off the ground.