For Mac:According to Gimp download page:
the 2.8.2 version GIMP runs on OSX natively. No X11 environment is required.However, I found it relatively tricky. If you download the dmg file from here (v2.8.2) which comes without any add-ons then you are in troubles. Since it doesn't have any add-ons in the package, then you have to manually install them by yourself, in this case, UFRaw plug-in.
On the other hand, if you are familiar with MacPorts and you can just type the command in terminal "sudo port install gimp" and MacPorts will install all related packages for you including UFRaw. Just so you know this version of Gimp must be run in X11.Well, I have tried to install Gimp via MacPorts but the UFRaw plug-in didn't work for me and to be honest I don't like the GUI with X11. So I would not recommend you to download and install both packages. However, if you download this package from here which is built by Simone Karin Lehmann, then your life will be much easier. It comes with many useful add-ons including UFRaw plug-in. I found it was much easier for me to start with Gimp.