GasSimTM is a web app offered by ScienceBySimulation. GasSim is a gas law modeling and simulation tool, intended for undergraduate and high-school students in chemistry or physics courses. GasSim includes a database of gas properties for 76 gases. It provides a simple interface that allows the user to vary the temperature, volume or the number of moles of a selected gas, and evaluates the pressure as a function of the chosen simulation variable using three different gas models: the ideal gas model, the van der Waals model, and the Peng-Robinson model. GasSim is designed to serve as a virtual lab that students can use to experiment with and develop a deeper understanding of ideal and real gas laws.