1. Create new new empty image using gimp with the same dimensions as your Blender frames
2. Press CTRL+ALT+O to open blender frames as layers
3. Press NAME to sort them by descending alphabetical order to have the layer show up in the right order
4. Select first frame, hold SHIFT and select last frame and click Open
5. Click File – Save as.. (or Save a Copy..) and choose a filename ending with .gif
6. Click Save As Animation – Export and finally OK
