This presentation explores GPU acceleration of parallel multigrid solvers for Finite Element simulations on commodity based clusters. The integration, based on a mixed precision iterative refinement scheme, is minimally invasive in the sense that it requires no changes to application code: Hardware acceleration is completely encapsulated from the application level. We demonstrate good scalability and accurate results for the prototypical Poisson problem and applications in linearised elasticity and incompressible Navier-Stokes simulations. Finally, we derive a performance model to accurately estimate the achievable speed-up.