  The discovery of a non-thermal radio ring of low surface brightness about one
degree in diameter has been recently reported around the location in the sky of
the Calvera pulsar, at a high Galactic latitude. The radio properties point to
it likely being a new supernova remnant (SNR), G118.4+37.0. We report an
analysis of almost 14 years of observations of this region by the gamma-ray
Large Area Telescope onboard the Fermi satellite. We detect extended GeV
emission consistent with the size and location of the radio source, which
confirms the presence of relativistic particles. The spectrum of the
high-energy emission is fully compatible with an origin in the same
relativistic particles producing the radio emission. These features and its
similarities to other isolated SNRs establish this source as the remnant of a
supernova. A simple model of the non-thermal emission from radio to GeV
energies resulting from leptonic emission from electrons produced by the SNR is
presented. G118.4+37.0 and other similar isolated remnants could be part of a
radio-dim SNR population evolving in low density environments showing hard GeV
emission of leptonic origin. Future deeper surveys in radio and gamma-rays
could discover new members of the group.
