The first-order magnetostructural phase transition in Gd5(SixGe12x)4 alloys with x<0.5 and the related entropy change DS are analyzed from high-field magnetization curves up to 23 T, and differential scanning calorimetry up to 5 T. The variation of the transition field Ht with the transition temperature Tt is discussed for fields up to 23 T for the x<0.5 range. From these data, the ratio dHt /dTt , which is related to the strength of the magnetoelastic coupling, is obtained as a function of x. We show that DS obtained from the ClausiusClapyeron equation and the Maxwell relation are equivalent for all compositions (0