The File Sequencer plugin is licensed under the GPL.
The source code is available here.
gimptool --install fileSeq.c will compile the plugin and install it in your local Gimp plugins directory.
The plugin appears in the Xtns menu as File Sequencer....
fileSeq is a simple plugin for Gimp 2.x which makes it easy to work on a numbered sequence of images, one at a time. When started, it attaches to one of the current images and displays the image filename. (The image cycle button allows you to cycle through the open images to select the correct one.) Pressing the next button will save the current image (if it needs saving), then look for a number in the file name, increment that number, and load that file in place of the current image. back will decrement the file number.