Gonzalo finally gets a chance to jest with Sebastian and Antonio. After his speech about ruling the island, the counselor asks King Alonso if he was listening (line 169). Alonso responds that Gonzalo is talking foolishly, that he is talking about nothing.
Alonso’s response has a double meaning: (1) that the speech is merely foolishness and (2) that the speech does not have any meaning to a king in the real and practical world. Despite his position as the ruling authority in Naples, even King Alonso cannot change the very foundation of society. Gonzalo agrees with his king, and comments that his speech was intended as a merry jest for the sake of Antonio and Sebastian, who always “laugh at nothing” (173). Gonzalo’s response is a criticism of these two lords. The counselor is implying that they are silly and childish.
When Antonio responds that they were not laughing at nothing but they were laughing at Gonzalo, the counselor again outwits them. He says that he is nothing to these lords (that he is of no consequence or importance to them). So, they are still laughing at nothing. However, both Antonio and Sebastian have such little regard for Gonzalo that his witticism does not affect them. Sebastian even comments that Gonzalo’s witty remark has fallen flat (line 178: like the flat side of a sword is ineffective in wounding an enemy). Sebastian is saying that Gonzalo’s remark does not bother them in the slightest.
Gonzalo does make one final comment in criticism of two men. He tells them that they “are gentlemen of brave mettle” (179). The word mettle means courageous, and Gonzalo is sarcastically implying that Antonio and Sebastian are anything but courageous in their verbal attack on the counselor. The word mettle is also a pun for metal (a homonym). A sword is made of metal, and thus Sebastian and Antonio are attacking Gonzalo with a sword made of speech, of empty air. Gonzalo then concludes that Sebastian and Antonio are so quarrelsome that they would even find fault and argue with the moon if they saw occasion. Of course, criticizing and attacking the moon would be a futile and impossible gesture; and so the counselor is implying that the remarks of the two lords are pointless and ineffective.