I have used Gimp for noise reduction and it is quite good.  I believe it has a "noise" filter but I have not used it and don't know how well it works.  I usually just use one of the tools to select the area I want to apply noise reduction to and the select the Gaussian Blur filter (or effect - not sure which pull down menu it its in) you can then modify the amount of blur.  This works quite wll I have found.   fred

You should only apply the noise reduction where it is needed. Otherwise you lose too much detail where you want it.  Some of the NR programs (I'm most familiar with Noise Ninja) apply noise reduction and sharpening to help avoid this but I think this would be difficult to achieve by hand using GIMP.  fred

I have been using NeatImage applied to the entire image.  You can download and use a pretty good version of it for free. I later paid the fee and upgraded myself ...

That being said, I've learned here on MSG that its better to identify only areas with noise, select those areas and use Noise Reduction on only the selected area. I am in the process of learning these techniques myself ...

Mark