GEM which stands for "Graphics Environment for Multimedia" is a tool for visuals. With GEM it is possible to generate and manipulate 2D and 3D graphics and animations, process & modify media like images & videos and generate particles.
This manual will explain the main objects that comprise GEM, and the basic techniques you will need to get started creating your own GEM projects.
A good start to get an idea about the various possibilities what can be done with GEM is to take a look at the examples & help patches that come with GEM. They can be can be accessed via the Pd Help Browser (in the Help menu, under "Browser..."), under "examples/Gem" or have a look at the GEM manual in "manuals/GEM."
GEM is a Pd Library and comes ready to use as a part of Pd-extended. It was initially written by Mark Danks. Some of the past and current GEM developers are IOhannes Zmölnig, Chris Clepper, James Tittle(tigital), Cyrille Henry.