For a novel, or a series of novels, imagine if there were a sequel in which a major character from the earlier books has grown into an immortal and hideous hybrid with an awesome, dangerous entity from the earlier books. And that the hybrid rules the universe and spends half of the novel talking about philosophy and politics and the other half rolling on people and squashing them.
Then compare this sequel with the actual fourth novel in the series (or the last book in the series or the novel itself, if there is no fourth sequel).
The test is named for God Emperor of Dune, the fourth of the original Dune series by Frank Herbert, in which Leto II becomes an enormous sandworm hybrid who rules the universe and in his spare time talks about politics and rolls on people and squashes them.
The exact form of the comparison is up to you. As God Emperor of Dune is the ne plus ultra of freaky sequels, the test can be used to benchmark weird or disappointing sequels, but it could also be used to imagine interesting non-existent texts, such as a sequel to Moby Dick in which Ahab returns as a half-whale, half-man who rules the seven seas.