Emacs (and hence org) will use whichever gnuplot is found in your $PATH, assuming you're on Linux (you did not specify). You also, for babel, need the gnuplot mode. I don't understand why you removed the gnuplot package as it does not have the emacs mode; that is provided by the separate gnuplot-mode package.
I don't use greek letters so cannot comment on that aspect. -- Eric S Fraga via Emacs 27.0.50, Org release_9.2.3-327-g3375f0
