Under GGA, the structural, electronic and magnetic properties of single-wall (8,8) GeC nanotubes filled with iron Fen nanowires (n=5, 9, 13 and 21) have been investigated systematically using the first-principles PAW potential within DFT. We find that the initial shapes of the Fe5@(8,8), Fe9@(8,8) and Fe13@(8,8) systems are preserved without any visible changes after optimization. But for the Fe21@(8,8) system, the initial shapes are distorted largely for both nanowire and nanotube. The binding processes of Fen@(8,8) systems are exothermic, and Fe5@(8,8) system is the most stable structure. The pristine (8,8) GeCNT is nonmagnetic and direct semiconductor with a wide band gap of about 2.65eV. Projected densities of states onto different shell Fe atoms show that the separation between the bonding and antibonding d states is reduced as going from the core Fe atom to the outermost shell Fe atom. The spin polarization of the Fen@(8,8) systems and free-standing nanowires are higher than that in bulk Fe. And the spin polarization generally decreases with the number n of the Fe atoms increasing for both the Fen@(8,8) systems and free-standing nanowires. Both the largest spin polarization value itself and not more decrease with respect to value of free-standing Fe5 nanowire suggest the Fe5@(8,8) system could be of interest for the use in electron spin injection. The magnetism is mainly confined within the inner Fe nanowire for these combined systems. More importantly, the Fe5 nanowire encapsulated inside (8,8) GeCNT is under the protection of the GeCNT to prevent from oxidation, thus may stably exist in atmosphere for long time and can be expected to have potential applications in building nanodevices.