The magnetic phase diagram of the Gd - Lu alloy system has been studied in the multicritical region using ultrasound velocity and attenuation measurements and neutron diffraction. A phase diagram has been drawn up on the basis of these measurements. The phase diagram has much in common with that of Gd - Y including the existence of a ferro I phase in which a net c-axis moment coexists with a paramagnetic distribution of spins in the basal plane. For Lu concentrations greater than 23.5% a helical antiferromagnetic (HAF) phase exists. At low temperatures all of the samples studied exhibit a canted ferromagnetic structure. The HAF-to-canted-ferromagnetic transition occurs when the pitch of the helix approaches zero and there is a small temperature range () over which canted ferromagnetic and HAF domains coexist.