beginGL() now returns a PGL object that is part of the Processing core and exposes only a subset of the OpenGL calls (basically those that are part of OpenGL ES 2.0). But you can still retrieve the JOGL object that gives you access to the entire GL API. It is just a member of PGL:

Thank you for your fast answer!

But I've still got some problems.

If I try to access the JOGL object like this:

1. GL gl = pgl.beginGL().gl;                // JOGL's GL object

I am still not able to make all the GL calls, that were working in 1.51.

1. ...
   
2. gl.glBlendColor(1-alpha*i/lines, 0.5-alpha*i/lines, 0.2+alpha*i/lines,
         alpha/i/i);

This is due to changes in JOGL itself. Processing 1.5.1 used JOGL 1.1, while Processing 2.0 uses JOGL 2.0.

Thanks very much!