Dragon Gralat? No such gralat, At least not in Classic does such a thing exist. Which judging by your profile pic you most likely are referring to.
The highest gralat value type is a Rainbow gralat. Which hits at 10k value at minimum. all gralat types and so on can be found in the graal city bank as I'll post images of below.
Is it possible graal tried to slide in a little secret gralat type that is extremely rare and almost no one has ever obtained it? It is possible. But very unlikely. The highest gralat value i have ever seen i think was close to 120k by this Italian guy about 1 year ago. And it was still indeed Rainbow. If a Dragon Gralat does infact exist and graal just hasn't updated the book or told anyone. It is likely so high that it is just pretty unlikely such a thing is reasonable to aim for given time spent and the annoying messages. Which makes it only really possible to get for Macro/bot users who use a aimbot or however they still seem to macro....which now just makes it depressing because it almost seems like classic screwed everyone over who doesn't macro now by putting in a anti-macro system...that was suppose to stop people from using macros...?