Get it cooking temperature hot, apply a liberal amount of oil ( think swimming, acts as a lubricant) apply a fair amount of pressure to the grill brick, work the entire surface. I always do the edges first, then work the field in a circle 8 pattern. The brick will leave behind a lot of grit and gum up, again make sure there is plenty of oil. When you think its clean, scrape it all off with a flat scraper and do it again. Scrape, wipe with dry paper towels until you think all the grit is gone, Then wipe with a damp cloth towel until clean.
Ditch the chemicals........ Those should be only used to get stuff off that a brick won't, usually around the perimeter where cooks don't want to spend the time cleaning properly. This stuff is also taking your surface down to bare metal.