Modification of the magnetic properties in a solid-state material upon external stimulus has attracted much attention in the recent years for their potential applications as switches and sensors. Within the field of coordination polymers, gas sorption studies typically focus on porous solids, with the gas molecules accommodating in the channels. Here we present a 1D non-porous coordination polymer capable of incorporating HCl gas molecules, which not only causes a reordering of its atoms in the solid state but also provokes dramatic changes in the magnetic behaviour. Subsequently, a further solid-gas transformation can occur with the extrusion of HCl gas molecules causing a second structural rearrangement which is also accompanied by modification in the magnetic path between the metal centres. Unequivocal evidence of the two-step magnetostructural transformation is provided by X-ray single-crystal diffraction.