You can ask any Gen Con HQ if there is any place for open gaming. As they have the schedule of what tables are in use when, they can steer you to a table that won't be used for a while. That's part of what they are there for. The hotels are a better bet than the rooms in the ICC. At the Union Station, we can usually find you a place.
I just wanted to highlight this response. A common criticism of Gen Con is there is no opportunity for open gaming, but this is most definitely not true. As mentioned in this thread, you have 3 primary choices:
1) Just ask at the nearest HQ for an unused table. They will direct you to a table you can use that won't have events starting at it for the time you're looking to play. This will get you comfy seats and a lot of table space. And, of course, it is free - other than needing a badge.
2) Gaming in the Library. Yes, it's not free. But you're guaranteed a seat and table space. Primary downside is that everyone who wants to play in there needs to have library tickets. You are free to play your own games in the library for as long as you like. As always, the library allows re-entry privileges for the duration of your ticket, but once your ticket time has expired you won't be coming back in (duh) but on the other hand, they don't kick you out.
3) Gaming in restaurants/hotels/etc. This is my personal choice. Things we've purchased in the dealer hall we'll take to, say, The Velocity Lounge at the JW, we'll order some beer and appetizers and play there. Many of the hotels are entirely okay with gamers setting up and playing on their tables. In the JW it's no problem pretty much anywhere there's a table, but I can't speak for the other hotels.
So....lots of options to play games wherever, whenever you want. Option #1 probably being the _least_ known of the bunch.