How well do we know the value of G?

G is the constant (well, as far as we know) of nature whose value is
known with the least precision. How well do we know it? Presumably
only Cavendish-type experiments can measure it directly. Other
measurements of G, particularly astronomical ones, probably actually
measure GM, or GMm. In some cases, those quantities might be known to
more precision than G itself.

Suppose G were to vary with time, or place, or (thinking of something
like MOND here) with the acceleration in question. Could that be
detected, or would it be masked by wrong assumptions about the mass(es)
involved?

Just as an example, would a smaller value of G and correspondingly
higher masses be compatible with LIGO observations?