Once a year I give my grates a clean with a commercial oven shelf cleaner pack - consisting of plastic bag into which the grates go, with a large bottle of caustic solution. The instructions say 6 hours minimum, but I leave for 24 hours, and then hose down, and all the gunk just falls off.
The reason I do this is that the grills tend not to be used for 3-4 months during the winter, and although I try to keep them clean, there is frequently some growth by springtime. The rest of the year, I am generally cooking frequently enough for this not to be an issue.
This year, I used two packs of cleaner - one had the grates from my Traeger and GMG, the other was used to clean the grease trays. Came up brilliantly - I wouldn't say "shiny", but definitely clean.
