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Structure, magnetic and transport properties of YMn6Sn6-xGax (0 ≤ x ≤ 0.6) compounds with a HfFe6Ge6-typestructure were investigated. It was found that the Ga substitution leads to a contraction of the unit-cell volume. Atransition from an antiferromagnetic to a ferromagnetic (or ferrimagnetic) state can be observed for samples (0.1 ≤x ≤ 0.2) with increasing temperature. The antiferro-ferromagnetic transition for samples with x ≤ 0.2 can also beinduced by an exteal field. The required field is very low, and decreases with increasing Ga concentration. MoreGa concentration (x ≥ 0.3) leads to the samples being ferromagnetic in the whole temperature range below the Curietemperature. The Ga substitution weakens the interlayer magnetic coupling between the Mn spins. Correspondingto the metamagnetic transition, a magnetoresistance as large as 32% under a field of 5 T was observed at 5 K for thesample with x=0.2.