Thin cat and a gray mouth

Hi, I brought a new cat. he has a gray furr, and his paws are also gray. And when I brought him home, I noticed that his mouth is even gray? Is it normal for a cat to have a gray mouth or a bit dark? He's also very thin, how could I get him to the healthy weight?

Are you saying his gums are gray?  A cat's gums should be pink, not gray.  His lips might be gray, or black even, in the case of a black cat , but not his gums.

To fatten up a cat, you need to feed him higher calorie food.  What are you feeding him...canned or dry food, or both?  A good quality canned food, with perhaps an egg yolk mixed in a few times a week will help pack on the calories.  But canned food varies vastly in calories, so you just need to make sure you are feeding him one that does not skimp in them. 

BUT, cats should be on the thin side.  Better to be thin than overweight.  Not too thin.  Have you taken him to the Vet yet?  Always when you get a new animal, they should have a checkup, and your Vet should be able to tell you what his optimal weight should be, AND have a look at his mouth and tell you if there's a problem.