I needed to remove the moire patterns of a scan. After a quick search on google I found this descreen plugin.
Unfortunately this plugin also requires the FFT gimp plugin, which is old and requires compiling and that page describes installing it for Windows.
This in turn was a bit of a problem because the FFT plugin has a few dependencies that aren't adequately documented.
To successfully build the plugin you need to first install libgimp-dev and libfftw3-lib packages from the standard repositories (debian/ubuntu)
# sudo apt-get install libfftw3-dev libgimp2.0-dev
Follow this by extracting the tar archive somewhere and doing
# make && make install
Once this is done, grab the descreen.scm file from the first link and plop it into ~/.gimp2.8/scripts
After starting gimp up there should be a "descreen" option under Filters->Enhance
