Reassemble the grill, then turn it on to its highest temperature setting and close the hood. There may even be a cleaning level on your burner dials. After a few minutes, open the lid and scrape the grates vigorously with a non-bristle metal grill brush. One I particularly like is the $15 Grill Bristleless Scraper from Taylor. This brush has multiple surfaces and edges to attack and remove grime from grill grates. It even has a handy bottle opener.
Check the fuel lines for visible defects. If any fuel lines have unmistakable damage (like rips, holes, cracks), replace the parts immediately before your next use of the grill. Also be sure that any fuel lines have a smooth trail. You don’t want any of your fuel lines to be bent, because that will prevent the gas from flowing correctly. Check the exterior of the gas tank for any damage as well; things like dents, erosion, punctures, or any evident signs of damage. If you find areas that have obvious damage, you could potentially have a gas leak.