What's the meaning of "Dear old Pinafore" like in Maury's sentence: "I know–with intellectual lyrics that no one will listen to. And all the critics will groan and grunt about “Dear old Pinafore.” In page 21, when speaking of Gloria, Anthony asks Maury: "Sort of left-over"? What exactly does he mean by that? And what about the expression "human nutmeg" in page 74?

In Chapter 1, Anthony Patch is sitting together with his friends Dick and Maury at a table in a rooftop cafe. Dick, a successful writer, expresses his desire to pen a musical comedy. Maury, for one, is unimpressed:

"I know—with intellectual lyrics that no one will listen to. And all the critics will groan and grunt about “Dear old Pinafore.” 
Maury thinks that any musical comedy written by Dick is likely to be a little on the highbrow side, something which wouldn't have mass appeal. But it wouldn't even find critical acclaim, either. Even a highbrow musical comedy would still be looked down on by the critics. They'd much prefer a comic opera such as H.M.S. Pinafore by Gilbert and Sullivan.

Maury tells Anthony about Gloria, the young lady he met at the Abercrombies' tea. Though not quite a debutante, she did come out in Kansas City two years previously. This piece of information leads Anthony to make the "left-over" remark. What he's suggesting here is that perhaps Gloria was on the shelf, that she hadn't managed to find herself a man a full two years after she made her debut in society.
Maury continues chatting to Anthony about Gloria, casually mentioning the fact that she talked about her skin and the kind of tan she'd like to get. This leads Maury to get nostalgic about the impressive bronze tan he used to have two years ago. Anthony starts laughing, imagining what Maury might have looked like. He thinks that Maury would've resembled nothing so much as a "human nutmeg." By this he's referring to nutmeg, a spice noted for its dark brown color.