  We study spectral diagnostics available from optical spectra with R=17000
obtained with the VLT/Giraffe HR15n setup, using observations from the Gaia-ESO
Survey, on the gamma Vel young cluster, in order to determine the fundamental
parameters of these stars. We define a set of spectroscopic indices, sampling
TiO bands, H-alpha core and wings, and many temperature- and gravity-sensitive
lines. Combined indices tau (gamma) are also defined as Teff (log g) indicators
over a wide spectral-type range. H-alpha emission-line indices are also
chromospheric activity or accretion indicators. A metallicity-sensitive index
is also defined. These indices enable us to find a clear difference between
gravities of main-sequence and pre-main-sequence stars (as well as giant
stars): the (gamma,tau) diagram is thus argued to be a promising
distance-independent age measurement tool for young clusters.
  Our indices were quantitatively calibrated by means of photometry and
literature reference spectra (from UVES-POP and ELODIE 3.1 Libraries, and other
datasets), over a wide range of stellar parameters. The ability of our indices
to select peculiar, or other rare classes of stars is also established.
Finally, our gravity determinations support the existence of an older
pre-main-sequence population in the gamma Vel sky region, in agreement with the
lithium depletion pattern of those stars (abridged).
