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The magnetic properties of spinel GeNi2-2xCo2xO4 systems in the range 0≤x≤1 are studied by mean field theory and high-temperature series expansions. The nearest neighbouring and the next-neighbouring superexchange interactions J1(x) and J2(x) are evaluated for these systems in the range 0≤x≤1, by using the first theory. The intra-planar and the inter-planar interactions and the exchange energy are deduced. The second theory is applied in the spinel GeNi2-2xCo2xO4 systems, combined with the Padé approximants method, to determine the magnetic phase diagrams (TN versus dilution x) in the range 0≤x≤1. The obtained theoretical results are in agreement with experimental data obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility γ and the correlation lengths v are deduced.