Phase transitions between paramagnetic and commensurate or incommensurate antiferromagnetic states have been investigated in several gadolinium compounds (GdCu5, GdGa2, GdNi2Si2, GdCu2Si2) by means of specific heat measurements. The exact shape of the specific heat anomaly at the transition temperature reflects the type of magnetic arrangement involved. A mean field model quantitatively accounts for such a behaviour.