Well... at risk of being pedantic, the Colts don't determine who uses Lucas Oil Stadium (although quite obviously they'd be the 800lb gorilla as far as scheduling during the football season). The Capital Improvement Board of Managers of Marion County Indiana is listed as the stadium's management. I don't know if it's them or the "ICCLOS" group that actually does the day to day task of booking events. But the point is that it's not the Colts who manage it.
Granted, without some sort of tacit approval from the Colts, Gen Con on the field of LOS would likely not happen. But back in 2009 the Colts pretty much gave general approval in an open letter to the community:
"It is also important to note that, from the very beginning, Lucas Oil Stadium was designed to be a multi-use facility to accommodate many users, not just the Colts...... While we are immensely proud to be able to call Lucas Oil Stadium our home field, the fact of the matter is the Colts only use the stadium a maximum of nineteen days a year; and the facility is available for use by virtually anyone else the remaining 346 days."
http://www.wthr.com/article/colts-letter-to-fans-on-lucas-oil-stadiumSo as long as it doesn't conflict with a home game, the Colts say 'no problem'. And to be frank, the ICCLOS would be insane to try and preempt a football game. But since this year's date won't conflict, the field is open and available. We'll have to wait and see about future dates, but 2017 is quite obviously already decided.