GNU ps is an alternative solution to the messy ps in procps (currently 3.2.8). This ps under Linux tries to be compatible with many BSDs and Unixes, while having GNU long options (starting with --). The options' behaviors are weird and they are not well documented at all.
The purpose of GNU ps is to eliminate all the messes the backward compatibilities with BSDs and Unixes (who cares?) brought to ps.
Eventually, this will be a clear yet powerful ps under GNU/Linux.