My dad has attended this conference, they have a very strict schedule and I know he has never visited the parks while there for the convention. Some may bring their families but I think you'll actually find it may lower attendance and it will certainly lower the use of convention hotel buses as Gartner hires private busses to take conventioneers to their events.
The only place that's really affected by this is the Epcot Resort Area, which sees lots of signs and booths set up for the conference with many attendees moving from hotel to hotel for different events. That area may seem a bit more crowded than usual, but other than that, it doesn't affect WDW much.
We've stayed at an Epcot resort during the symposium week several times and other than seeing conventioneers around the resort, they don't affect park attendance very much. During the day they are busy with their meetings and during the evening they seem to get together mostly at the resorts and venues outside the parks.
I have attended this conference the past 2 years and will be attending again this year. There are about 13,000 people (between Gartner and the attendees). The dolphin, yacht and swan are the most impacted. The park crowd impact is not an issue. On one night they hold a conference event at one of the parks. Last 2 years it was HS and Epcot accordingly. They will close the park early for that night (about 8). I don't think they would ever do MK. My guess is it will be AK this year.