GOMC: GPU Optimized Monte Carlo for the simulation of phase equilibria and physical properties of complex fluids. GPU Optimized Monte Carlo (GOMC) is open-source software for simulating molecular systems using the Metropolis Monte Carlo algorithm. It supports simulations in a variety of ensembles, which include canonical, isothermal–isobaric, grand canonical, and Gibbs ensemble. GOMC can be used to study vapor–liquid equilibria, adsorption in porous materials, surfactant self-assembly, and condensed phase structure for complex molecules. GOMC supports a variety of all-atom, united atom, and coarse grained force fields such as OPLS, TraPPE, Mie, and Martini. The software has been written in object oriented C++, and uses OpenMP and NVIDIA CUDA to allow for execution on multi-core CPU and GPU architectures.