Most of the Gartner attendees are there only during the days of the symposium.
We were there last year during that week and while we saw them around the Epcot resorts, they didn't impact Epcot during the day.
They reserve a bloc of rooms at one or two of the Disney resorts, as well as the resort where the Symposium is being held, but not entire resorts. If the Epcot resorts are sold out during October, the popularity of Food & Wine Festival has much to do with it.