gThumb has a plug-in system for extensions. Some standard features are implemented as extensions and are supplied with the standard distribution. Users may write (and share) additional extensions.
You can enable or disable extensions using Edit ▸ Extensions. Some extensions will have Preferences that can be modified from this dialog.
This manual is written with the assumption that the standard extensions are enabled. If you seem to be missing a feature, check to see which extensions are actually enabled.
If you would like to write your own extension, please visit https://wiki.gnome.org/Apps/gthumb for development information. An example extension is provided in the source code, which you can base your own extension on.
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