Take a look here:
I'm pretty sure Gen Con is going to used the spaces on the event level, primarily the halls and rooms on the north side (based on space sizes in the press releases). When I've attended Colts games, most people enter on Street level, one floor up. If they have seats at the 100 level, they walk into the entry plaza, which is above the meeting halls, then walk down into their seats.
The CIB planned for simultaneous access. I imagine that if there is a year with a conflict, there may be restrictions or partitioning of the underground tunnel, but both events can take place. It would be a lot like when the RCA Dome field was right across the hall from the TCG room and both were in use. They put up some ropes and had some guards there to make sure you didn't accidentally use the field doors on that day.
If there really is an unresolvable conflict, an event blocking a pro game on the schedule is not unprecedented. In Chicago, the Bulls and Blackhawks have 2-week road trips while the Ringling Brothers circus uses the United Center. Banker's Life Fieldhouse in Indy has some similar constraints.