If you've been on the IPP in Elmhurst or Villa Park, that is crushed limestone. The GWT is almost entirely crushed limestone. There is a 5 mile section that is paved west of the Fox River.
If you're headed to Elgin then you'll actually switch from the GWT to the IPP Elgin Spur at Prince Crossing Rd. The Elgin IPP Spur turns to asphalt at the Stearns Rd/Dunham Rd trail junction. The pavement continues all the way to Crystal Lake. There is an short unpaved segment about a half mile long just south of Middle St that is peppered with gravel so be careful.
If you have the time, check out the trail north of Elgin. The IPP changes to the Fox River Trail and then the Algonquin Trail. Different names, same river trail. The ride from Elgin to Crystal Lake is one of my favorites.