GEGL filter
GEGL filters are also called "GEGL operations" or "GEGL plug-ins". You may see any of these naming, though they usually refer to the same thing.
Such filters will appear automatically in the list of filters available from the "GEGL Operation" dialog. What makes these very powerful is that they provide automatic on-canvas previews, with split view, and they are not specific to a version of GIMP (your GEGL operation will be usable in GIMP 2.10, 2.99, or even any other software using GEGL if they have a generic operation interface).
For a proper introduction and an up-to-date creation process, we advise reading this 2022/2023 tutorial by Liam Quin in 3 parts:
Installing GEGL filters
Apart from the standard operations bundled with GEGL, it will search for custom operations in:
Folders listed in the GEGL_PATH environment variable if it exists, overriding all other standard paths.
Or user data directory.
User data directory
Location: $XDG_DATA_HOME/gegl-0.4/plug-ins/
Defaults to $HOME/.local/share/gegl-0.4/plug-ins if XDG_DATA_HOME environment variable is not set
(see the XDG Base Directory Specification).
Location: %LOCALAPPDATA%\gegl-0.4\plug-ins\
As defined in local application data, e.g.
C:\Users\<UserLogin>\AppData\Local\gegl-0.4\plug-ins\
Location: ~/Library/Application Support/gegl-0.4/plug-ins/