Rated for gruel
Last time I looked, gruel was a thin porridge, often made of oatmeal (it's easier to make with oat flour, by the way) or some other grain. It's cooked until the grains are basically disintegrated and sometimes strained out; it was Ben Franklin's favorite breakfast when he was a young man in London learning the printer's craft; he liked it cause it was cheap and filling, and the place where he got it from served it with pepper and bread crumbs.
It was, of course, the food of the poor....and those with digestive problems (which is why I know how to make it!)
The urban dictionary says it can be great and cool combined, but the context here seems to be one of something negative.
I don't really want to read it to find out. I might find it...well.....gruelling.