I haven't used GAP, the GIMP Animation Package, to make a .gif in years. The OpenSCAD animation in "Replacing the Cardboard Box" was made by saving animation frames in OpenSCAD, then importing the frames as layers into GIMP. To split part of a video into .gif-able frames in GIMP requires the GAP add-in. Anyhow, the ancient GAP v2.6 still (sort of) works in the latest version of GIMP. Very simple Youtube instructions here.