There is a version of your image with captions
Which tells us that all of these are radio signals, on various frequencies.
The main measurement conducted is continuous, very accurate measurement of the distance between the two spacecraft. This is based on the travel time, and probably the phase of the radio signals between them, compared to the very stable "clocks" (referred to as ultra-stable oscillators) they each carry. The analogue between them is the one being measures, and the digital one is used to keep the clocks in sync and so on.
To understand how this translates into a gravity measurement, consider a region of slightly higher gravity (eg above a mass concentration). When the leading probe approaches that region it will be accelerated, and will move a little further ahead of the trailing probe. When the trailing probe reaches that region it will speed up as well and the movement will stop. When the leading probe leaves the region it will be slowed and they will move closer together. With care, this effect allows the gravity map to be recovered from ranging data.
I just found this source which gives actual accuracy for the measurement of the distance between the spacecraft of 30nm (roughly one millionth of an inch) over a five second period.
This cites a technical paper (Klipstein et al) for how the range measurement works, which gives a lot more detail and explains the exact roles of all the radio signals.