Opaquer Tool
 

The opaquer allows you to make pixels in a layer opaque, thus restoring erased pixels to their visible state. Using the right mouse button (or holding shift) makes the pixels slightly more opaque than they were, as dictated by the level setting. Using the left mouse button makes the pixels completely opaque.

This tool is pressure sensitive when using a graphics tablet; the pressure varies the width of the tool tip.

You can select a brush-tip mode for this tool.