While I don't necessarily trust the "leaks" that have happened, there are a couple of things that make me suspicious that there may indeed be no Gyms in Sun & Moon, first of which being that in the gameplay footage we've seen so far, we haven't seen any locations that could be gyms. In the very first trailer for X & Y, we saw the insides of buildings that were undoubtedly Gyms (Coumarine, Santalune and Anistar), but this time... nothing.
Another thing to take account of is the map of Alola, which to me does not contain 8 plausible locations that could be Gyms. I suppose it's possible that not every building in the region is shown, but certainly the in game footage of the Honolulu-based city is one-to-one with the map in terms of buildings.
At the end of the day, these observations are certainly nothing conclusive, and if we get a reveal of a Gym or Gym Leader the idea is obviously debunked, but I can't shake the feeling that they might not exist in this game. This could all be unfounded and I may have overlooked things, but I can't help but be suspicious.