You can use an onion to clean and season the grill. Cut an onion in half using a standard kitchen knife, then stick your BBQ fork into the back of it and wipe vigorously across the hot grill. The onion has a surprisingly caustic effect, removing food deposits and residue while also flavouring the grill.
Do not use harsh chemicals to clean your BBQ. Harsh chemicals can leave a residue, which can affect the taste and smell of the food the next few times you use the BBQ.