Pankaj - there are two distinct ways (that I can think of) to do the acknowledgement. The first is that in any paper, presentation, or proceedings you write - make sure to mention what different software you used. The other way that comes to mind is that each of the products will always take contributions (whether financial or time). You could become a member of one of the teams and contribute via documentation, testing, or even coding.

Of course! It is an absolute must! G-power software is open-source and free yet you should provide full reference when you use it for calculating power in your research!

Hi, yes, you need to cite open source software as well you cite any other resource that you use. If that software impact your results (in non trivial way), example, a numerical computational package as Octave, you can use this FAQ of octave page http://wiki.octave.org/FAQ#How_can_I_cite_Octave.3F and when you need an specific toolbox use the citation command to display how to cite, example, "citation geometry".

For gnuplot this a reference for v4.0 http://onlinelibrary.wiley.com/doi/10.1002/jae.885/abstract or you can cite the manual of your version.