To be fair, the quoted sentence itself sets up the jargon used: "... a center of strong, or positive, gravity ..." Think of it as setting up an analogy's names

To be fair, the quoted sentence itself sets up the jargon used: "... a center of strong, or positive, gravity ..." Think of it as setting up an analogy's namespace. Or just refer to it as gauge gravity, which is what they've described (where zero is a reference to a nonzero absolute value).