I live in the Chicago area, and yes it is FLAT!. And other than the DesPlaines river trail, which is under water 1/2 the year, there is no gravel roads or trails. I rode the Prarie Path, crushed limestone, for years on skinny sew-up tires, and that was 50 years ago when it was just starting to be developed. Lots of dirt, mud, and irregularities.
Now I ride the Green Bay Trail, mostly crushed limestone, on my old road bike with...skinny sew-up tires. No issues whatsoever. And they are nice and fast on the road.
Gearing, I find my low gear of 44/21 handles anything in a 50 mile radius and beyond. So a gravel bike with its ultra low gears is totally unnecessary in this area.
My recommendation, get an old steel road bike, glue on some skinny sew-ups, get some tall gearing, get out on Sheridan road....and ride like bloody hell !