GTA 5 was released 5 years after GTA 4, and even during that time we had two amazing single player DLCs and a separate release for both DLCs in one disk. It's been 5 years since GTA 5 and we've had one cancelled single player DLC, and only one 'meaty' update to GTA Online, Heists. Other than that they've milked GTA:O with a steady stream of low effort cosmetic upgrades, cars and game modes that recycle existing assets (And most new conent is created by the players using map editors). Since we've had no news at all of another GTA title in the time it's taken them to release an entire game previously, I think it's safe to say we wont see GTA 6 till the next gen.