Just to add to the excellent answer above, that is is first and foremost the gom-jabbar is a test, to which almost the entire opening of the book is devoted.
"Sleep well, you sly little rascal," said the old woman. "Tomorrow you'll need all your faculties to meet my gom jabbar."
"Paul . . . " Jessica took a deep breath. ". . . this test you're about to receive . . . it's important to me."
Second the reason for the poison test is to filter human from animal. It's purpose is embedded in the BG mind-body lesson Paul was using before bed:
. . . one does not obtain food-safety-freedom by instinct alone . . . animal consciousness does not extend beyond the given moment nor into the idea that its victims may become extinct . . . the animal destroys and does not produce . . . animal pleasures remain close to sensation levels and avoid the perceptual . . . the human requires a background grid through which to see his universe . . .
Pride overcame Paul's fear. "You dare suggest a duke's son is an animal?" he demanded.
"Let us say I suggest you may be human," she said.
Third it consists of two parts:
"I hold at your neck the gom jabbar," she said. "The gom jabbar, the high-handed enemy. It's a needle with a drop of poison on its tip. Ah-ah! Don't pull away or you'll feel that poison."
The needle and the poison at the tip. So to be a human gom-jabbar we would need two things.
- The needle - This is the Bene Gesserit themselves they will hold the 'poison' at the throat of the species to filter out humans from animals.
- The poison - This is the Kwisatz Haderach for whom the BG toiled for 90 generations, filtering and pruning the bloodlines. Paul himself.
"Ever sift sand through a screen?" she asked.
The tangential slash of her question shocked his mind into a higher awareness: Sand through a screen , he nodded.
"We Bene Gesserit sift people to find the humans."
The BG had long recognized the problem and dangerous possibility of human extinction. They wanted a solution (their Kwisatz Haderach) but on their terms. Save the human race yes, but to their overwhelming and total advantage. They would control the destiny and direction of all of humanity, the ultimate totalitarian system. He was going to be the poison that they drove into the flesh of the human race. Their human gom-jabbar!