I use a steel wool pad and barkeeper's friend.
Do this outside.
What I do is tale a large zip close freezer bag and put the stove grate in it, add 1/2 cup ammonia, seal it closed and let it set overnight. It is the fumes that do the work. The next day and still outside, put on rubber gloves and remove the grate, rinse well and let dry. I usually do 2 at a time so I have 1/2 stove available.
I put my stove grates in the dishwasher. I have been cleaning this way for many years and happy with results.
What about black ceramic grates?
With a stove cleaner which you can purchase at your local grocery store