GIMP's lack of content-aware fill features was the reason why Photoshop won me back. However, the Rsynthesizer plugin adds ...

The plug-ins should be easy to install on Linux and Mac systems, and a few offer pre-compiled Windows versions. For help getting your plug-ins working, check out the official documentation.

Once installed you need to download BIMP, the Batch Image Manipulation Plugin, and install it (see Wikibooks GIMP/Installing Plugins for the details). Again, easy-peasey.