In act 2, scene 1, Antonio and Sebastian do more than criticize Gonzalo; they are actively mocking him. This scene takes place after the shipwreck, when these characters have washed up on Prospero's island, and morale (naturally) is at a low. Gonzalo is seen here speaking optimistically about their situation, stating that they should count themselves fortunate that they survived the shipwreck at all. All the while, Antonio and Sebastian are seen constantly belittling his words.
When discussing Gonzalo, Antonio, and Sebastian in this scene, consider the intent that motivates their actions. Ultimately, I think there is a strong argument that Gonzalo's words are overly optimistic and unrealistic, perhaps even dangerously naive, and if Antonio and Sebastian had merely criticized Gonzalo on this account, then their actions might have been defensible. But such a reading runs contrary to the scene as Shakespeare wrote it.
The truth of the matter is that, regardless of whether or not you would agree with his words, Gonzalo is trying to be constructive. He is attempting to keep up their spirits by suggesting that things are not as bad as they look. Antonio and Sebastian, on the other hand, are not offering anything constructive at all; instead, they are mocking Gonzalo for their own shared amusement. It amounts to nothing more than pettiness.
This should not be surprising, considering that Antonio has already been established as the villain of the play, who had previously betrayed and usurped his own brother, Prospero, and very soon, under Antonio's influence, we will see Sebastian attempt to do the very same thing against his own brother, Alonso, the current king of Naples. These characters are not well-meaning or benevolent, and these interactions should be read with that in mind.