Undo/Redo

Main Menu: Edit > Undo and Edit > Redo
Shortcut: Ctrl-Z and Ctrl-Alt-Z
Toolbar buttons: and

Undo

Reverses the last changes made to the active image.
The number of steps, or undo limit that the program stores is 3 by default, but it can be a set to any other value (provided that the system has enough memory) in the Preferences window.

Undo can remove painting or drawing operations, color alterations, filter effects, etc., but it cannot undo any modification that has been saved.

To remove all changes made to an image since it was saved use Revert to saved.

Redo

Re-applies the last action or command on which you have used the Undo command.