I picked up a 5 day parking pass this year from Indianapolis Event Parking.  The outdoor lot on Kentwood was around $23 a day. 

I could not find a way to buy a pass online through Chance Events.  Has anyone else bought their parking yet?  

It sounds like you secured a 5-day parking pass at a good rate of $23 per day, but had trouble buying it online through Chance Events. Has anyone else found a way to purchase parking passes or had similar issues?. pk555game.pk

You can contact Chance Events Parking directly by phone or email. Here's their Facebook page with the contact information: https://www.facebook.com/ChanceParking

Guessing Gate10 isn't handling parking anymore?

The TL;Dr is Gate Ten allegedly breached contract last year before GenCon 2024 (this is why so many people were uprooted from their parking in 2024) and ceased running the parking lot. 

Indianapolis Event Parking is the official group now that runs some of the lots as of earlier this year. They are not affiliated with GenCon, just run the parking. The Horseshoe parking that is south of the stadium across McCarty street is now ran by Indianapolis Parking Company and it does not look like they have GenCon on their list of events. Maybe it will be added later. 

More info....

If you hate all of this and want to gamble, Denison Parking manages all of the official parking for the Indianapolis Convention Center and Lucas Oil Stadium parking (ICCLOS- disgusting... I know). Their website probably doesn't show GenCon 2025 yet or it might not at all since I doubt GenCon will partner with them ($$$). You can always check as the date gets closer.

If all else fails... ParkWiz or JustPark and book each day individually. Denison Parking actually recommends those two for special event parking... but again, GenCon might not be listed on their websites so you might have to enter in the dates manually. 

I live just outside of Indianapolis.... so have to constantly search for parking when events are going on.