The actual quote, to put this bit in context, from Stave One of the text is:
“You don't believe in me,” observed the Ghost.
“I don't,” said Scrooge.
“What evidence would you have of my reality beyond that of your senses?”
“I don't know,” said Scrooge.
“Why do you doubt your senses?”
“Because,” said Scrooge, “a little thing affects them. A slight disorder of the stomach makes them cheats. You may be an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato. There's more of gravy than of grave about you, whatever you are!”
This statement is Scrooge's way of belittling the apparition of Marley that is before him. He does not believe in ghosts, so there must be some other explanation for this vision of Marley that is before him. He suggests that his disagreeable stomach, after his eating something that does not sit well in his digestion, has affected his senses so that Marley has appeared before him.
Scrooge is presented here as a very mundane, very practical fellow. It will take the convincing of Marley and Three Spirits for Scrooge to begin to believe in the power of the things that he cannot perceive with his five senses alone.
Dickens explains Scrooge's "gravy" comment with these words:
Scrooge was not much in the habit of cracking jokes, nor did he feel, in his heart, by any means waggish then. The truth is, that he tried to be smart, as a means of distracting his own attention, and keeping down his terror; for the spectre's voice disturbed the very marrow in his bones.
So his play on words -- Grav-y / grave -- is also a joke meant to try to get the upper hand in a situation that is, in fact, putting Scrooge in the position of being terrified. Could it be that he perceives that something amazingly life-changing is on the horizon for him? If so, he is correct. Scrooge is in for the alteration of a lifetime.