Ive looked into this answer extensively. The tires that come on the gladiators dont fit most aftermarket rims of the same diameter. Most aftermarket 18" rims are 9.5" wide (the cool ones at least) The tires that come on the gladiator are good up to 9.0 rims......could they fit? Yes, but you change the ride characteristics and increase the possibility of the tire separating.
I would be more worried about higher speed separating than low speed gravel.
Its the only reason I haven't upgraded my rims yet, I want to get the money out of my tires that came with the truck before essentially disposing of them....probably keep one as the spare.