Curves is a powerful tool found in The Gimp, it can be used for many things, such as fixing a colour cast.
To use the curves tool, go to <Image> Tools > Colour Tools > Curves and you should see a window similar to this:
You can adjust the "Value" channel, which will brighten/darken the image depending on whether you drag the line up or down. Down will darken, up will lighten.
You can also adjust the Red, Blue, Green, and Alpha channels.
If your picture has a green colour cast, open up Curves, switch to the green channel, then move the line toward the bottom a little bit, until the cast is gone.
You can also switch the curve style, from smooth to jagged.