Intermetallic compounds R2Fe17 are perspective for applications as permanent magnets. For the practical usage, these systems must have the Curie temperature Tc much higher than room temperature and, preferably, an easy axis anisotropy. Nowadays, the highest Tc among the stoichiometric R2Fe17 materials is 476 K, which is not sufficient. There are two possibilities to increase Tc: substitution of Fe ions with non-magnetic elements or introduction of light elements into interstitial positions. In this work we have focused our attention on the substitution scenario of rising the Curie temperature, which was observed experimentally in the Gd2Fe17-xGax (x=0, 3, 6) compounds. In the framework of the LSDA approach the electronic structure and magnetic properties of the compounds were calculated. Ab initio exchange-interaction parameters for all nearest Fe ions in the Fe sublattice were obtained. Employing the theoretical values of exchange parameters, the Curie temperatures Tc of Gd2Fe17-xGax were estimated within the mean-field theory. The obtained values of Tc agree well with experiment. Also the LSDA computed values of total magnetic moment coincide with the experimental ones.