We Will All Laugh At Gilded Butterflies
To go along with tthakkar's answer, the word "gilded" can mean 'deceptively pleasing' or 'covered with blood'. When both of these definitions are applied to the two other daughters, it shows that Lear is 'laughing' at them because of their 'false fronts', but it also holds the darker meaning that Lear's and Cornelia's blood is on the heads of the other two daugthers.
As the other responders remark, "gilded butterflies" refers to the courtiers, along with all the connotations that they suggest are contained in the metaphor. It seems to me what is very i here, however, is not just what they refer to but also Lear's attitude toward them: because he is with his daughter, Lear will "laugh" at the butterflies, doing so as the two of them pretend "they are God's spies." In prison, their position becomes privileged, not punitive. They will see more, know more, and live more deeply than they did before. And they will do so on the side of God. What before had been problematic--these courtiers--will now be inconsequential. The "gilded butterflies" might indeed be free outside the cage of prison in which Lear and Cordelia will reside, but since prison will allow them "to sing," they will be more free than those insignificant, superficially beautiful insects (which is what butterflies are) flittering about aimlessly. Of course we need to remember that Lear is quite mad in this final scene, but perhaps it is the kind of madness that Plato speaks of, the kind that permits special knowledge, a greater view of life.
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http://www.enotes.com/kl/q-and-a/what-does-laugh-gilded-butterflies-mean-thanks-5059