GSPs come in many colors and patterns and each is unique, it is one of my favorite things about GSPs! I will do my best to explain the differences here.
Roan V. White
Let's start with the basics, GSPs are either roan or white, this is their base pattern. If you look at the skin, under the hair, a roan dog will have gray/blue skin, while a white dog will have pink skin. Please note where you are looking though, the skin under ticking and patching can be gray/blue, even though the dog is white. Sometimes you can look at the paws, a roan dog's paws are nearly always gray, where as the white dogs often have a mix of pink and gray pads.
Patching is different from ticking in a number of ways. Patches are the large solid spots that are seen on the animal. Unlike ticking, and even roaning, you will see patching at birth and it will stay on the dog forever, at grow proportionally. Patching is very genetic, to include the amount and the location. Buster, my main stud, has a patch that is on his tail base, slightly to the right, many of his children have a variation of this patch in this same location.
Ticking is the little spots on a dogs coat. It can change the entire look of a dog. It can make a white dog look roan or it's absence can make a dog look the purest of whites. I find ticking can either be light or dark, not that the AKC has a way to specify this, however, notice that some of the ticked spots are really only on the skin, with very few colored hairs, many ticked spots have white hairs growing out of them, however the darker ticking actually has pigmented hair growing out of it. I refer to this difference as lightly ticked or dark ticked, but again, this is just my own distinction.