Hi downlow. There is no fundamental limit to the frequency a photon can have, so as qraal suggested if you give us your source it might help you get a better answer.
I'm speculating, but what you could be referring to is the Greisen–Zatsepin–Kuzmin limit (commonly just the GZK limit) that postulates a limit to the
cosmic ray energy, as seen in particles striking the Earth's atmosphere. Note that cosmic rays are not photons (and hence not gamma rays) but are in fact very fast moving particles such as protons or other small nuclei. The origin of this limit is due to the cosmic microwave background. Cosmic rays above the limiting energy would interact with CMB photons producing multiple lower energy particles ( I can't remember the exact mechanism at play). This continues until the cosmic ray is below the threshold for this type of interaction.
Strictly speaking, this limit applies only to cosmic rays traveling distances much greater than the size of our galaxy, meaning that we
could see cosmic rays over this limit if there was a source strong enough within our own galaxy. We think that the very highest energy cosmic rays are produced in Active Galactic Nuclei (which our galaxy does not currently have) and so if we did see cosmic rays above this limit it would be interesting; either there is some stellar sized source capable of making such high energy particles, or for some reason high energy particles can travel long distances. To date not conclusive observations of particles above this limit have been made, although there has been some controversy around the question.
Note that
time dilation effects are strictly a function of distance - i.e., the Hubble flow. There is no intrinsic gravitational time dilation from the star itself.
is not very helpful. Time dilation can occur for any number of reasons, including in intrinsic effect due to photons climbing out of the gravitational potential of the star or whatever else is emmitting a photon. When trying to work out the energies that particles have when emmitted from various astrophysical events in principle this instrinsic effect needs to be taken into account. In practice how important that is in comparison to cosmological effects will depend on the nature of the event and the distance from us that it occurred. I'm not sure in practice how important this is for Supernovae, but certainly the gravitationl time dilation is observed in the observations of AGN.
