The Ga-stabilised $Sm_2Fe_{17-}$type alloy can hardly be disproportionated under ordinary HDDR condition. The HDDR characteristics of Ga-substituted $Sm_2Fe_{17-}$type alloy were examined, and, in particular, the effect of particle size on the disproportionation of the Ga-substituted alloy was investigated in detail. The reaction characteristics of the $Sm_2Fe_{17-}$type alloys with or without Ga-substitution with methane (CH4) gas are also examined. The Ga-stabilised $Sm_2Fe_{17-}$type alloy was able to be disproportionated significantly on heating up to 80$0^{circ}C$ under hydrogen with normal pressure. The particle size influenced significantly on the disproportion-ation of the Ga-substitute alloy, and the materials with finer particle size (<40 ${mu}{ extrm}{m}$) was fully disproportionated on heating up to around 80$0^{circ}C$ under hydrogen gas with normal pressure. The Ga-substituted alloy has a very sluggish recombination kinetics with respect to the alloy without Ga-substitution. The $Sm_2Fe_{17}C_{x-}$type carbide was stabilised significantly by the Ga-substitution for Fe in the parent alloy. While the $Sm_2Fe_{17}C_x$ was disproportionated below 80$0^{circ}C$ the Ga-stabilised $Sm_2Fe_{14}Ga_2C_x$ carbide remained intact even on heating up to 80$0^{circ}C$.