I will start you off.
Examples of frames are here,
viewtopic.php?f=9&t=11486A couple are already linked:
FrameFactory.py (amazingly still works)
viewtopic.php?f=9&t=11486&start=10#p155526Pin-Stripe Border (not really a frame)
viewtopic.php?p=27323#p27323Gradient Frame
viewtopic.php?f=9&t=5725edit: Get the zip and gradients from above link but the script does need a little updating for Gimp 2.10 , I made a few tweaks, dare say it need more. script attached.
Many are on Paul Shermans site under scripts > edges
https://www.gimphelp.org/script210.htmlespecially
https://www.gimphelp.org/edges_picture_frame.html and
https://www.gimphelp.org/edges_add_matte.htmlOthers, From the Beautify plugin (only partly works) G'mic (Frames and samj, let you look through those) and stickit.scm for example is in the gimpscripts archive.