So this is my friend and my first time at GenCon. We've done other conventions in the past, mostly anime conventions. They all had almost all of their events as a part of the badge - as in, there were few things you would have to pay for to go to, on top of the admission price, but it was "first come, first serve" basis. For every 10 things we did there were probably 1 or 2 that we were turned away from due to the room being full. There are a LOT of events listed with a ticket price of $0, that still have 50-100, sometimes 200 spaces still open. Is a ticket necessary for those? If I show up at the door without a ticket for a free event that still has a bunch of seats open will I be turned away, or, for free events is it more just a reservation type thing?
You will be guaranteed a seat with a ticket. Collecting tickets also lets Gen Con know how successful an event is, to see if they need the same amount of space in future years...
Most free events don't fill up, and you'll be able to join without a ticket. Even some that do fill up (like The Stink) still let people in without tickets after everyone with a ticket has entered. No one will turn you away if there is room.