I always thought gruel was a slightly thinner version of porridge.
But I found this recipe in the “Essex Cookery Book” of 1930 by K. A. Willson and Margaret Hussey, and it turns out it’s almost homeopathic porridge, so tiny is the quantity of oatmeal – 1 tablespoon to a pint of water or milk.
It’s featured here as invalid cookery, which admittedly sounds like a perfectly fine use for it. But those having to actually survive on versions of this were pretty hard done by. No wonder Oliver Twist wanted more.
(As a side note, you may be interested to learn that the tradition survives these days in the form of the old favourite Horlicks which is technically gruel – http://en.m.wikipedia.org/wiki/Gruel)