I use a stiff wire brush to clean the grates. For the heavy stuff I use a SS wire brush from the hardware store (welding supplies area). For lighter stuff I use a regular grill brush that has a scraper end on it. Yes, brushing and scraping will remove some of the seasoning, at first. But, in time, that won't be a problem. Keeping the grates clean, even if it does remove some of the seasoning, is important. In time those cast iron grates will be almost non-stick, if you keep them clean as the seasoning builds up.
I like to scrap the grates while they are hot to get off the heavy stuff. Sometimes both sides of the grates need to be done, especially after butts or chicken halves.
After scraping them, just spray or wipe with cooking oil. Next time I fire up, I'll brush/scrape them again and apply another coat of oil when they are hot.
For the grill body, I spray, followed by a paper towel wipe to get off the excess oil. All you need for that is a thin film of oil. Too much oil can get gunky, for lack of a better word. It's better to build up the seasoning with thin coats of oil.
For the interior of the CG main chamber and the charcoal pan, I keep those lined with foil. Makes clean up so much easier.
Two sheets of the wide heavy duty foil, lined up front to back will take care of the chamber. The narrow HD foil is the right width to go under the grate in the main chamber charcoal pan. Then, tear a length in half to cover the sides of the pan above the grate.
To clean up prior to foiling, use a cleaner such as Greased Lightening to clean up that charcoal pan. Use anything you have handy to scrape under the charcoal pan grate. Something like a yardstick works good. Spraying with a water hose is fine. Once you have it clean, rinse and dry thoroughly. Then, apply a coat of oil, followed by the foil.
The foil will generally last through several cooks, especially when you're using the offset. Just replace, as needed. BTW, I also foil the ash tray in the SFB for the same reasons.
Hope this helps.