GM

GM is the gravitational parameter μ. It varies from system to system; per system, we need it whenever calculating a Keplerian orbit. M is the mass of these orbits' barycentre. G is Newton's "constant".

It turns out that G is poorly constrained. A lot of the error-bars I see around orbital dynamics in extrasolar systems come from our inability to figure G, thus the mass of the suns in question.

ETH Zurich have recently constrained it... 2.2% higher than the consensus up to today. Or, at least - they promise they will constrain it. Some of these papers aren't in publication yet. And we're still screwed on "dark-matter" elsewhere.

Without agreement on basic cosmologic parameters (admittedly better outside gravity), the space is opened up for Big-Bang skeptics. And for the temptation for censorship of same which - I hope, obviously - does not help anybody.

Our standard-model is getting challenged elsewhere, if I'm understanding the Turtle on the muon.