I'm not qualified in this area but grey or black gums can be a serious issue. We had a dog who ate rate poison once and required emergency vitamin "K" injections at the vet - a full series for a number of days and the way they could tell it was serious at first was by checking the color of the gums of the dog. Pink to grey, dark grey being worst. Could it be possible that your animal got into rat or mouse poison? Just thinking of the worst just in case. Good luck.
http://www.virtualvet.com/petowners/poisonfacts.htm