The present paper is addressing the static neutronic analysis, and a code-to-code validation, of water-steam injection accidents for three different Gas-cooled Fast Reactor (GFR) core designs. It is assumed that this type of accident can occur as consequence of either the rupture of a pipe in the main steam generator or a leak in the decay heat removal heat exchanger. The analysis is focused on fast-spectrum, helium-cooled systems currently being developed and investigated in the context of the Generation IV International Forum (GIF) and the 6^th Framework Program of the European Union. More specifically, two 2400 MWth GFR cores and a small-size 50 MWth GFR demonstrator have been analyzed and systematically compared for a large range of water-steam densities from 0 (dry core) to 250 kg/m³ within the core. The neutronic analysis was performed using both the deterministic system code ERANOS-2.1 and the Monte Carlo method MCNPX-2.5 code, in association with modern nuclear data libraries, i.e. JEF-2.2 and ERALIB1 (adjusted library) for ERANOS, and JEF-2.2 for MCNPX. First, simulations were performed based on cell models and then with whole core representation, in order to ease the code-to-code comparison. Based on the core analysis, the k_eff-value for the two GFR core designs is seen to first increase with the water-steam density, and then, beyond 40-100 kg/m³, to decrease monotonically. On the contrary, for ETDR, the k_eff-value increases throughout the analyzed water-steam density range. Globally, a good agreement is obtained between the deterministic and stochastic results, the discrepancy being in the range of a few hundreds of pcm. Additional investigations have been conducted on effects such as the neutron spectrum softening, leakage reduction and the contribution of structural materials. It has been observed that non-conventional materials, such as tungsten, play a major role and help counteract the positive reactivity effect. Finally, the analysis has shown the suitability of ERANOS/ERALIB1 for the core analysis of the GFR cores, characterized by a softener neutron spectrum compared to Na-cooled systems.