In this work, the Gas Source State Estimation (GSSE) problem in non-trivial geometries is approached by combining a non-stationary gas dispersion Partial Differential Equation (PDE) with in-situ gas measurements under the assumption of known flow. The GSSE problem is formulated as an optimization problem with PDE constraints and solved efficiently using the adjoint method. The approach is simulatively validated on a 2D problem with laminar flow around a circular obstacle and data from gas dispersion simulations. Thereby, a single Gaussian gas source could be localized accurately for most trials with randomly placed sensor.