Garmin really does not do that for shows but some of their distributors might run some kind of special on their own. Occasionally Garmin does do special deals but those are well documented and not necessarily tied to any show.
I haven't seen the 2019 Oshkosh rebate list yet, but last year we had a $50 off rebate for GDL 50/51/52 units purchased at the show from one of our dealers, so hopefully that will happen again.