In this study we combine tethered bilayer lipid membrane electrical impedance spectroscopy (EIS) and molecular dynamics (MD) simulations to characterise the structure and cation-selectivity of ion channels created by pore-forming peptides [1]. While the ability of viral fusion or antimicrobial peptides to form ion-conducting pores is well documented, the structures of these pores are poorly defined. We characterize the size and multimeric structure of pores formed by GALA; a 30-reisdue peptide, pH-sensitive peptide. Specifically, we used a number of alkaline earth cations and the large organic cations, choline and triethanolamine, to estimate the pore size by measuring the concentration-dependent changes in membrane conduction. This data was combined with MD simulations to estimate the multimeric state of GALA pores. We report that in planar phospholipid bilayers, GALA pores likely consist of six peptide monomers rather than eight to twelve monomers, as previously reported. We further demonstrate, for the first time, that GALA exhibits cation selectivity, which is based on ΔGhydration of the ions.