The invention relates to a former for fabricating a gas composite material casing for a gas turbine, the casing being obtained by forming fiber reinforcement as superposed layers of a fiber texture and densifying the fiber reinforcement with a matrix. The former has two cheekplates and an annular wall with an outside surface presenting a profile that corresponds to the profile of the inside surface of the casing that is to be fabricated, the annular wall of the former including removable fastener means for fastening a fiber texture layer and serving to hold the first layer of the fiber texture mechanically while the fiber texture is being wound on the former.