Try using a spray-on oven cleaner.
Scrape off as much junk as you can.
Get a large (32 gallon) trash bag, put your grills inside, spray them on both sides to fully saturate them, and close the bag using a rubber band or twist tie. Leave it overnight to let the chemical do its job. Remove the grills and rinse several times with hot water.
I think you will be very pleased with the results.
To maintain them after cleaning - after cooking, close your grill's lid and turn up the heat to at least 500 F to burn off any cooking residue. Then turn off the gas and let the grill and racks cool down overnight. Next time you use them, give them a quick brushing.
And each time you grill, when the oven and racks are hot enough to cook on, wipe them down with a bit of olive oil on a cloth to keep food from sticking.