It posits that a rather repetitious writer wrote out a set of Greek Notes of Jesus tradition, sometimes re-using part of a story he had already recorded to make more effective another story. The collection as a whole was arranged roughly in what appeared to be chronological order. Each synoptist independently used the Greek Notes, editing the material he selected, to try and produce a book which could be read as a continuous account of Jesus - a gospel. On this view, the synoptic gospels are therefore three separate edited selections of material from the same set of notes in Greek. Not at all.
Hypothesis is very different.
Lessing s hypothesis fails because the verbal similarities in Greek between the synoptic gospels are strong and detailed in many places. Independent translators do not produce such close similarity in wording. So Lessing s hypothesis can be ruled out on these grounds alone. On the other hand, the GNH accounts easily for the similarities in Greek in the synoptic gospels, since it posits that each synoptist used the same Greek set of notes. To solve the Synoptic Problem we need the simplest possible documentary hypothesis which is consistent not only with the pattern of story dualities but also with all the observed synoptic phenomena.