Gimp Gradient Extrapolate is a plugin for Gimp 2.8.
It fills gaps or removes objects, both of which are surrounded by a gradient like smooth image structure (e.g. sky).
Refer to the wiki to see how to use it.
Note, since this plug-in is written in python and involves optimization problems and non-filter operations, this script's runtime can be rather long, depending on the size of your image selection.
Consider donating to this project: https://sourceforge.net/projects/gimp-g-e/donate