The TmMn 6 Ge 6 - x Ga x compounds (x=0.2, 0.4, 0.6, 0.8 and 1.0) have been investigated by magnetization measurements and neutron diffraction. The TmMn 6 Ge 5 . 8 Ga 0 . 2 compound orders antiferromagnetically at 471 K in a colinear magnetic structure characterized by a doubling of the c axis. Below T t =220 K, a helimagnetic structure takes place. At 2 K, the spiral plane lies in the (101) plane with μ M n =2.02(5) μ B and μ T m =6.27(9) μ B . The TmMn 6 Ge 5 . 6 Ga 0 . 4 compound orders antiferromagnetically at 402 K. From 300 to 42 K, it is characterized by a helimagnetic arrangement with a helical plane lying in the (101) plane. Below 42 K, a conical structure takes place and the direction of the ferromagnetic component is almost aligned along the c axis. The Mn and Tm moments refined at 2 K are μ M n =2.21(5) μ B and μ T m =6.82(8) μ B . The TmMn 6 Ge 5 . 4 Ga 0 . 6 compound orders antiferromagnetically at 379 K. From 300 to 124 K, it is characterized by a helimagnetic arrangement with a helical plane close to the (001) plane. Below 124 K, this compound becomes ferrimagnetic with moments almost aligned along the c axis. The Mn and Tm moments refined at 2 K are μ M n =2.19(4) μ B and μ T m =6.89(7) μ B . The TmMn 6 Ge 5 . 2 Ga 0 . 8 and TmMn 6 Ge 5 Ga compounds order ferrimagnetically at 360 and 359 K, respectively, and a moment reorientation takes place at 73 and 56 K, respectively. At high temperature the moments are close to the (001) plane whereas at low temperature they almost align along the c axis in good agreement with the large coercive field reported at 4.2 K for this compound (H c ~20 kOe). The Mn and Tm moments refined at 2 K are μ M n =2.20(5)μ B [2.18(4)] and μ T m =7.01(8) μ B [7.01(8)], respectively. The evolutions of the moment magnitudes, magnetocrystalline anisotropy and interlayer couplings, as a function of the Ga content, are discussed.