A man that goes in for honour at Cambridge —i.e. a mathematical degree—is sometimes too bad to be classed with the lowest of the three classes, and yet has shown sufficient merit to pass. When the list is made out a line is drawn after the classes, and one or two names are appended. These names are in the gulf, and those so honoured are gulfed. In the good old times these men were not qualified to stand for the classical tripos.
“The ranks of our curatehood are supplied by youths whom, at the very best, merciful examiners have raised from the very gates of `pluck' to the comparative paradise of the `Gulf.' ” —Saturday Review.
A great gulf fixed. An impassable separation or divergence. From the parable of Dives and Lazarus, in the third Gospel. (Luke xvi. 26.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Gulf from Fact Monster: