tl;dr: Did you put it in ~/.gimp-2.8/plug-ins and set the executable bit ?
It’s been a while since I developed a script to automate tasks in GIMP. I figured I would do one for the repetitive tasks for creating a custom YouTube Thumbnail (more on that later perhaps). But my script wasn’t showing up in the Filters menu.
I had found the preference for setting the directory: Edit ? Preferences ? Folders ? Plug-Ins (not that GIMP treats python as plug-ins, not scripts); with the default user folder being ~/.gimp-2.8/plug-ins. But the plug-in dind’t show up.
Restart GIMP. Still nothing.
Ask on IRC. Double check the documentation (always a good idea). Aha!
Scheme and Python plug-ins are readable text files. C-language and Python plug-in files must have permissions set to allow execution.
chmod +x myscript.py later, and it registered!
Hope this saves someone the twenty or thirty minutes it took me to find this out!