In this letter we demonstrate the applicability of an augmented Gibbs ensemble Monte Carlo approach for the determination of the phase behavior of model colloidal systems with short-ranged depletion attractions and long-ranged repulsion. This technique allows for the first quantitative determination of the phase boundaries and ground states in such systems when the range of the attraction is a small fraction of the particle diameter. We demonstrate that gelation may occur in such systems as the result of arrested microphase separation, even when the equilibrium state of the system is characterized by compact microphase structures.