  In this paper we propose to use the measurement of the thermal Quark-Gluon
Plasma (QGP) dilepton spectra in the Intermediate Mass Region (IMR) of
heavy-ion collisions, as a new method to search for GeV-scale dark gauge bosons
(gamma' or Z'). Such light mediators are a common feature of light (i.e. low
mass) dark matter scenarios, which have been invoked to explain puzzling
signals in dark matter indirect and direct detection experiments. First we show
that a light gamma' or Z' will generate a resonant enhancement of the dilepton
spectrum produced thermally by the QGP, at an energy corresponding to the dark
gauge boson mass. Secondly, using data from the PHENIX experiment, we are able
to set an upper limit on the combined coupling of this new gauge boson to
quarks and leptons (independently of their vectorial or axial nature) chi_q
chi_e < 10^(-3) at the 95% confidence level for a gauge boson mass in the range
1.5 - 2.5 GeV. This result complements previous searches for new light gauge
bosons and probes a new region of the parameter space, particularly interesting
in the case of non-universal couplings to quarks and leptons. Prospects for the
discovery of such a boson by the ALICE collaboration are also discussed.
