Pull the "flavor bars" off, and if you can get the burners out (many are simply dry fit), the culprit will likely be under the burner, especially if you have no drain for the grease to run out. Heat rises, and it often doesn't get hot enough under the burners for the gunk to burn completely off. Folks often overlook the bottom of the grill, and it needs attention too. I've taken rags and mopped up the goop when it's viscous, a scraper when congealed, and shot a drain hole in the bottom with a collection bucket as well. Also, X2 what Beta said: after you've gotten as much out as you possibly can manually, fire that thing up high and let the rest burn off for 45 minutes or so.