A former for fabricating a gas composite material casing for a gas turbine, which is obtained by forming fiber reinforcement as superposed layers of a fiber texture and densifying the fiber reinforcement with a matrix, is disclosed. 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 includes a removable fastener device 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.