What is the difference between a Gesso and a Ground?
A Gesso can be considered a ground, but a ground does not have to be a gesso. A ground is a product that provides a desired surface on which to paint. GOLDEN makes two grounds: Absorbent Ground and Acrylic Ground for Pastels. The Absorbent Ground is applied to any gessoed support and mimics the absorbency of a good watercolor paper. Acrylic Ground for Pastels is a clear product with a gritty solid added to it to allow pastel to adhere to it. The vast array of Gels and Mediums in the GOLDEN line can be used in a similar fashion, depending on what the artist is looking for.

GOLDEN's gesso seems much thinner than other brands I've used. I'm sure my adding 25% GAC 200 made the gesso thinner and gave it less covering power, but even straight it was thin.
Our Gesso is not the thickest version on the market, and may have been part of the reason for the poor coverage. It does have a good amount of pigment in it, but we chose these pigments for their resistance to yellowing and brightness rather than sheer covering power. However, it is unusual you experienced the kind of poor coverage you describe. The 25% addition of the GAC 200 certainly didn't help for pigment load, but it also thins the product down even further. We have seen thin paints give poor coverage compared to thicker paints having the same amount of pigment. For example, Fluid Acrylics compared to Heavy Body Acrylics. Same pigments, same pigment load, but brush each one and the difference is night and day. If you have been used to thicker gessos and compared it to ours, thickness difference may be what you are noting. If you want to thicken the Gesso, blend with 10%-30% GOLDEN Heavy Gel Matte, and it should have similar coverage to what you are used to.

Can I sand black gesso or do I need white?
You can sand the white and black Gessos, they will just gum up a little more than our Sandable Hard Gesso. This can be lessened by making sure you allow enough time for the paint film to cure (at least 48 hours) before sanding. Additionally, you can blend in some of the GAC 200 to facilitate better sanding. The GAC 200 is a hard acrylic resin that lessens the gummy nature of the acrylics.

Is sanding between coats of Gesso absolutely necessary?
GOLDEN Gesso does not require any sanding, either in between coats or on its surface, unless you desire to do so to create a smoother finish. There is no concern of intercoat adhesion.