The hands-down, best method I've come across for cleaning flattop grills (after years of splashing caustic amounts of grill cleaner and other horrible chemicals on them), is oil and kosher salt. A flat top grill is like a cast iron pan. Once it's been seasoned well, it performs beautifully and doesn't take that much to maintain. My husband and I have been in this business all our lives and we use this on all the grills we cook on.
The grill needs to be hot for this method, but not "cooking" hot... like a few-minutes-after-you've-turned-it-off hot. Scrape off (with a grill scraper) as much of the residue of as you can. Pour a high-heat oil (like sunflower, grapeseed, peanut, etc) on the grill (depending on the size of the grill, but about 2 cups) and then sprinkle a very generous amount of salt (like a cup) on the oil. And then scrub with a grill brick. Initially, if you are cleaning the grill for the first time, it will take quite a bit of elbow grease, a griddle screen under the grill brick and maybe a few back-to-back cleanings, but you will be rewarded with a nicely seasoned, low-maintenance flat top.
Scrape the salt and oil residue with the grill scraper and then wipe the grill down with a clean cloth, dampened if necessary. It has been my experience that anything acidic, like vinegar or lemon juice or grill cleaner, is damaging to the grill surface and creates pock marks over time.
Hope this helps!