Hi, support for geometry shaders is not planned for Processing 2.x. As Amnon pointed out, the shader API follows the GLES 2.0 spec, which doesn't include geometry shaders. However, you can get the underlying JOGL objects and access all the GL functions for profiles 3.0+, which you can use to create and run shader programs with a geometry stage. The section "PGL interface" in this post might help.
As for the getTessellation() function, it returns a PShape object that contains all the geometry from the source shape, but stored as triangles.