Having worked on classical general relativity for his PhD thesis, Gibbons focused on the quantum theory of black holes afterwards. Together with Malcolm Perry, he used thermal Green's functions to prove the universality of thermodynamic properties of horizons, including cosmological event horizons. He developed the Euclidean approach to quantum gravity with Stephen Hawking, which allows a derivation of the thermodynamics of black holes from a functional integral approach. As the Euclidean action for gravity is not positive definite, the integral only converges when a particular contour is used for conformal factors.
His work in more recent years includes contributions to research on supergravity, p-branes and M-theory, mainly motivated by string theory. Gibbons remains interested in geometrical problems of all sorts which have applications to physics.