A couple things on this. Unfortunately we don't have any workouts or race simulation stuff to look at to get information here but assuming that you're in sub 2:50 shape.
Unfortunately you haven't given yourself much time to practice taking gels ahead of this. For your next time out, start implementing a fueling strategy during your long runs so you have that all locked down pre race day. Pick out a couple different types of gels/flavors you may think you enjoy. Test it first by having it about 45 minutes prior to your easy run. Basically do this to see how it responds with your stomach as it's settled. Better to have stomach issues during an easy run than a long run. If you found something you like try it out on your long run this weekend. Nothing you do less than 2 weeks out is going to make you any faster for race day.
Chicago fueling strategy suggestion:
I'm personally a fan of taking it every 5 miles or 30 minutes. You want to be replenishing those burned calories. It may feel all fine at 10 or 13 but when you get to 20 and you need those energy reserves, you're going to be in big trouble. Utilize the drink stations too, it ends up making the gels easier to take down.
Racing Strategy Suggestion:
Aim for a negative split. My typical strategy for these races are 10-10-10K. First three miles of the marathon are warming up and staying conservative, then settling into a comfortable pace. At 10 start moving down the pace to something that feels comfortable, then 10K leave everything out there. Use checkpoints for yourself to adjust race strategy if necessary. You're better off having a down mile early than blowing up and letting the race become a death march. At Columbus I negative split by going out in 75:42 and coming in with a 2:30:25. If the goal is to break 2:50, heading out just about a minute slower may be a good move (1:25:30 or 1:26:00). If it feels tough there, just hold on to that pace, if you feel good, enjoy blowing by people that thought "banking time" was going to pay off.
Good luck to you out there.