Grid Systems is a formalism for modelling population and ecosystem dynamics that combines features of membrane computing, such as rewrite rules and maximal parallelism, with a representation of space similar to that of Cellular Automata. Moreover, Grid Systems include features for the description of environmental events and of events that can be associated with frequencies and durations that can be either deterministic or stochastic. The combination of all of these features makes Grid Systems a comprehensive formalism for the modelling and analysis of ecosystems. This tool paper describes the implementation and the features of a simulator for Grid Systems. The simulator is equipped with a graphical user interface for defining and editing models of populations, and for simulating population dynamics and movement. The aim of this tool is to allow modellers to construct and analyse models based on a comprehensive and rigorous formalism such as Grid Systems with a friendly interface.