The Gravity Anomaly or Bouguer Anomaly Maps are no actual gravity maps.
They are rather a map of ground density, expressed as an excess or deficit of gravity caused by differences in density.
The maps are the result of measuring the gravity and subtracting all effects of oblateness, terrain shape, terrain height and so on. The result is a map of differences in gravity due to the structure of the material under the surface.
The image below compares the elevation, surface gravity and gravity anomaly maps of the Moon. The difference of the plots can be seen rather nicely around the two large craters close to the border of the maps:
From the elevation map we can see that both crates have a somewhat three-level structure: an deep inner part (blueish), an outer part with intermediate depth (greenish) and surrounding mountains (reddish).
If the underground was homogeneous, we might expect to see the same structure in the surface gravity plot: The deeper the terrain, the higher the gravity due to being closer to the more dense core of the Moon. But, there is only a two-fold structure: High gravity in the inner part of the crater, and then a somewhat constant gravity despite the elevation change.
This is reflected in the Gravity Anomaly plot: The ring with intermediate elevation has a negative anomaly (dark blue) while the inner part and the surroundings have a positive anomaly (red). That means that the outer ring of the craters is made up of less dense material. Not being a geologist I'd guess that this is due to material kicked up during the impact and settling in a more loose way when falling back down.
