Howdy
In Iris, load the image you want to get rid of gradients from, then go to Processing, go down to Remove Gradient (polynomial fit), tick balance background colour then hit ok, normally the other default settings work well as they are.
Iris is free though has a steepich learning curve, theres all sorts of hidden functions, even I am still discovering things. The main help pages are here
http://astrosurf.com/buil/us/iris/iris.htm
under tutorial.
Hope that is of assistance
Cheers
Scott
Scott