So simple you will wonder why you did not think of it yourself.
All you need is ammonia and a garbage bag.
To clean 4 grates:
Put 1 cup ammonia in a large garbage bag.
Tie it up and set in your laundry room overnight. Or put the bag outside on a warm sunny day.
The grime will just dissolve on it own.
DO NOT combine any other cleaning solution, just the ammonia.
Use a soapy sponge and hot water to rinse off.