What can I use to clean the metal grates on the stove?

Get a garbage large enough to hold the grates. Put a cup of ammonia in the bag and seal the bag. Let this sit for several hours. Take the grated out of the bag and wash in hot soapy water.

I let them soak in the sink with 1/2 c dawn and a dryer sheet (weird I know but it works). Let it soak for 6-8 hours (depending on how bad they are). When you're ready use the dryer sheets to wipe off the gunk - it should come off so easy you won't believe it.

https://www.mykitchenescapades.com/how-to-clean-stove-grates-no-scrubbing/

Put them in a plastic bag with a bowl of ammonia overnight, in the morning wipe clean.

I agree with Terry except I use 1/2 c. of ammonia. It works great for barbecues too. You can leave it on the patio overnight, and then hose them down before bringing them in, to remove most of the crud, or clean them in your laundry tub.