  A key observational prediction of Einstein's Equivalence Principle is that
light undergoes redshift when it escapes from a gravitational field. Although
astrophysics provides a wide variety of physical conditions in which this
redshift should be significant, till very recently the observational evidence
for this gravitational effect was limited to the light emitted by the Sun and
white dwarfs. \textit{Gaia}-DR2 astrometric and kinematic data, in combination
with other spectroscopic observations, provides a test bench to validate such
predictions in statistical terms. The aim of this paper is to analyze several
thousand main-sequence and giant stars in open clusters (OCs) to measure the
gravitational redshift effect. Observationally, a spectral shift will depend on
the stellar mass-to-radius ratio as expected from the theoretical estimation of
relativity. After the analysis, the obtained correlation coefficient between
theoretical predictions and observations for 28 (51) OCs is $a= 0.977 \pm
0.218$ ($0.899 \pm 0.137$). The result has proven to be statistically robust
and with little dependence on the details of the methodology or sample
selection criteria. This study represents one of the more extensive validations
of a fundamental prediction of gravity theories.
