"what a Gorgon is like..."
See in text (Act I)
"Gorgon" refers to any of three monstrous sisters from ancient Greek mythology who were so grotesque that anyone who looked directly at them would immediately turn to stone. The most famous Gorgon is Medusa, a terrifying creature with snakes for hair who was, unlike her sisters, mortal and was slain by the Greek hero Perseus, thus making her story a part of popular culture. In likening Lady Bracknell to the Gorgons, Jack characterizes her as a particularly detestable woman.
"the manna in the wilderness..."
See in text (Act II)
An allusion to Exodus 16 in the Bible, when Moses leads the Israelites into the wilderness and God feeds them with "manna" and quail from the sky. According to Exodus 16: 31, manna "was white like coriander seed, and it tasted like honey wafers." It's thin and flaky like a cracker and sees the Israelites through a very hard time in their history. Here, the Chasuble intends to use this story to say that God will provide for Jack in his moment of grief.
See in text (Act II)
In ancient Roman mythology, Egeria was a nymph who become King Numa Pompilius' trusted advisor and companion. Egeria purportedly advised him on the creation of Rome's laws and rituals, basing them on religious practices predating the formation of the Roman Empire proper. As a nymph, Egeria was also said to be very beautiful, which is why the Chasuble has made the comparison: he's infatuated with Miss Prism.
See in text (Act III)
A reference to Immanuel Kant, a famed German philosopher whose skepticism was highly influential in the intellectual community. Kant believed that nothing can be truly known or understood except that which we experience empirically in the natural world, and therefore that "truth" is a fallacy and one can never believe what others say. It happens that Gwendolen knows about this philosophical stance and chooses to believe Jack anyway. This decision seems to put to rest the tension between truth and lies, which have been major themes in this play.