We present a study of 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, with the purpose of classifying these stars and finding their fundamental parameters. We define several spectroscopic indices, sampling the amplitude of TiO bands, the H{alpha} line core and wings, and temperature- and gravity-sensitive sets of lines, each useful as a Teff or logg indicator over a limited range of stellar spectral types. H{alpha} line indices are also useful as chromospheric activity or accretion indicators. Furthermore, we use all indices to define additional global Teff- and logg-sensitive indices {tau} and {gamma}, valid for the entire range of types in the observed sample.