<p>Lamellar gel phase bilayers, a key component of lamellar gel networks (LGNs), confer unique stability and rheological properties to LGNs for pharmaceutical and cosmetic applications. The bilayers are typically composed of long-chain fatty alcohols (FAs) and surfactants with large polar headgroups. However, it remains unclear how the surfactant weight percentage (wt%) in the bilayers modulates bilayer properties. We perform molecular dynamics simulations of gel phase bilayers containing cetyl and stearyl FAs with various behentrimonium methosulfate (BTMS) wt%. We find that increasing surfactant content increases the area per lipid and bilayer thickness but decreases the lipid order parameter, all in a nonlinear fashion. The competing effect between bilayer thickness and lipid packing order leads to non-monotonical modulation of both bilayer stretching elastic modulus and shear viscosity by BTMS wt%. Our improved understanding provides a guideline for the formulation development and optimization in pharmaceutical and cosmetic applications.</p>