Though the more interesting part is that with such sensitivity you would likely be able to infer nearby masses moving around - shipsnor stealth planes for example.

Difficult to do over any significant range: gravity drops off with 1/r^2, gravity gradient like what they seem to be using here (easier to measure while in motion) drops off with 1/r^3. The variations in the earth are due to fairly large density variations in massive sections of the crust, and a ship and especially an aircraft is absolutely miniscule in comparison, even if the density contrast is higher.