  Inclusive jet cross sections for events with a large rapidity gap with
respect to the proton direction from the reaction $ep \rightarrow jet \; + \;
X$ with quasi-real photons have been measured with the ZEUS detector. The cross
sections refer to jets with transverse energies $E_T^{jet}>8$~GeV. The data
show the characteristics of a diffractive process mediated by pomeron exchange.
Assuming that the events are due to the exchange of a pomeron with partonic
structure, the quark and gluon content of the pomeron is probed at a scale
$\sim (E_T^{jet})^2$. A comparison of the measurements with model predictions
based on QCD plus Regge phenomenology requires a contribution of partons with a
hard momentum density in the pomeron. A combined analysis of the jet cross
sections and recent ZEUS measurements of the diffractive structure function in
deep inelastic scattering gives the first experimental evidence for the gluon
content of the pomeron in diffractive hard scattering processes. The data
indicate that between 30\% and 80\% of the momentum of the pomeron carried by
partons is due to hard gluons.
