Can gravitational waves diffract off massive objects?

It seems to be still a research question for specific situations , but the answer is yes, gravitational waves diffract. example this paper:

Emission of gravitational waves from binary systems in the galactic center and diffraction by star clusters

From the abstract:

The diffraction pattern of gravitational waves emitted by a binary system by a cluster of stars has been also analyzed. We remark that this is only a preliminary-theoretical work than can acquire more interest in view of the next-coming gravitational wave astronomy era.

in the paper:

it suffers diffractive effects while interacting with stars. Hence, by considering each star as a circular slit and applying the well known theory of wave diffraction, we can evaluate the expected diffraction patterns on the observer plane.

Other papers studying diffraction of gravitational waves here and here.