WebDec was originaly written so I could get at the webshots images on linux, webdec fullfilled this need but it didnt offer any graphical interface or integrate with browsers very well to automate image extraction.
The next project I started was webman. webman was initially designed to be called by the web browser when a webshots file was downloaded and call webdec to extract the images and then store them in a specific directory. If it was configured the image could also be set as your wallpaper.
Again there were problems with the way images were stored and people were asking for a GUI, instead of re-writing everything into a gui, webdec and webman were kept and gwebdec was built on top.
So today you can still download webdec individually, but the latest version of webdec is always included in the gwebdec packages. The updates included with gwebdec tend to be minor and specific to gwebdec, major improvements will be released as individual webdec packages.