To clean the gunk off the racks on your grill, wrap them in newspaper or paper towels and place them in a plastic garbage bag. Saturate the paper with household ammonia, tie the bag closed, and let it sit overnight.
The next day take the racks out—being careful to avoid breathing the ammonia fumes—remove the paper, and spray the racks down with a hose.
If needed, wash them in warm water with dishwashing liquid to remove any remaining residue. Rinse and dry.