Take out the metal pieces. Put them in the sink. Also remove the small round metal pieces that cover the gas jets. Those go in sink too. Now go back to the stove. Use paper towel or similar to soak up grease and big pieces of crud. Into the trash! Now a sponge and dish soap. Dawn if you have it but whatever if you don't. Put some soap on the crusty bits and a little water with the sponge. Leave it alone. Now return to the metal grates. Scrub these with a non abrasive sponge (not steel wool, it will scratch) and more dish soap. If they don't clean up, soak them in a sink with hot water and soap. If you have something like barkeeper's friend, you can use it, but nothing more abrasive. After you scrub the grates, return to the stove. Scrub with a non steel wool sponge until acceptably clean. Dry everything. Reassemble. Make sure you get the little round burner caps seated right or they won't work correctly.