The Collaborative International Dictionary of English v.0.48:

Gruel \Gru"el\, n. [OF. gruel, F. gruau; of German origin; cf.
   OHG. gruzzi groats, G. gr["u]tze, As. gr[=u]t. See Grout.]
   A light, liquid food, made by boiling meal of maize, oatmeal,
   or flour in water or milk; thin porridge.
   [1913 Webster] grueling

WordNet (r) 3.0 (2006):

gruel
    n 1: a thin porridge (usually oatmeal or cornmeal)

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "gruel":
   albumen, batter, bonnyclabber, breakfast food, butter, cambric tea,
   cereal, clabber, cobweb, cornflakes, cornstarch, cream, curd,
   dishwater, dough, dry cereal, egg white, farina, frumenty, gaum,
   gel, gelatin, glair, glop, glue, gluten, goo, gook, goop, gossamer,
   grits, gumbo, gunk, hair, hasty pudding, hominy grits, hot cereal,
   house of cards, jam, jell, jelly, kasha, lath, loblolly, matchwood,
   mere shadow, milk and water, millet, molasses, mucilage, mucus,
   mush, oatmeal, pap, paper, paste, porridge, pudding, puffed rice,
   puffed wheat, pulp, puree, putty, rail, rake, reed, rob,
   rolled oats, rope of sand, sand castle, semifluid, semiliquid,
   shadow, shaving, size, skeleton, slat, slip, soup, splinter,
   starch, sticky mess, streak, syrup, thread, treacle, vein, wafer,
   water, wheatflakes