I had always imagined that gruntles were something that no gentleman should be without, possibly an obscure component of mediaeval armour like the bit covering the elbows or perhaps something to put on your escutcheon.
Imagine then my disappointment on looking up disgruntled in my Compact Edition of the Oxford English Dictionary (The photographically reduced one which needs a magnifying glass).
v trans To put into sulky dissatisfaction or ill-humour: to chagrin, disgust. (from DIS + GRUNTLE)
sb 1. The snout of a pig, or other animal 2. A little grunt
v 1. To utter a little or a low grunt 2. To grumble, murmur, complain
Of course disgruntled may mean having had your snout or your little grunt removed, but I suspect it's a false negative meaning induced to grunt, grumble, murmur or complain.
All in all, I prefer my imagined derivation.