GBTUL (acronym for “GBT at the University of Lisbon”) is a freeware program that performs elastic buckling (bifurcation) and vibration analyses of prismatic thin-walled members. It implements the latest formulations of Generalised Beam Theory (GBT), a thin-walled bar theory that (i) accounts for local deformation and (ii) provides an advantageous representation of the deformation field, as a combination of structurally meaningful cross-section deformation modes.