U(2) gauge theory with Nf=24 massless fermions is non-interacting at long distances, i.e. it has an infrared fixed point at vanishing coupling. With massive fermions the fermions are expected to decouple at energy scales below the fermion mass, and the infrared behavior is that of confining SU(2) pure gauge theory. We demonstrate this behavior non-perturbatively with lattice Monte Carlo simulations by measuring the gradient flow running coupling.