I didn't look at the schedule but if you're talking about the big dinner, it's people only. There are dog sitters available if you need one.
Before last year, the Eisenhower was the popular hotel. Last year they didn't accept dogs, so we were spread out throughout the town. The Eisenhower is having dogs again this year and Nittany has rooms at a reduced rate.
I can't say if everyone will be there but it will probably be filled.
If you want to see Gettysburg, I can recommend two non-greyhound activities. I would try the horseback tour of the battlefield or the Segway tour. These are not part of the greyhound event.