The magnetic susceptibility of ferromagnetic GdM alloys (M=Cu_{1-x}Ga_{x}, Mg, Zn) has been investigated under helium gas pressure for temperatures above T_{C}. The evaluated pressure derivatives of the paramagnetic Curie temperature, dlnΘ/dP, appeared to be remarkably different for isovalent GdMg and GdZn compounds (-11.2 and -0.1 Mbar^{-1}, respectively). An analysis of the obtained dlnΘ/dP values for GdCu_{1-x}Ga_{x} alloys and results of ab initio electronic structure calculations have revealed the essential role of 5d electrons as the mediators of exchange coupling in ferromagnetic GdM compounds. The pressure derivatives of T_{C} were calculated by employing the modern mean-field theory, as well as the spin-fluctuation model. As a result, good agreement is found with the experimental values of dlnΘ/dP.
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