  The Herschel GASPS Key Program is a survey of the gas phase of protoplanetary
discs, targeting 240 objects which cover a large range of ages, spectral types,
and disc properties. To interpret this large quantity of data and initiate
self-consistent analyses of the gas and dust properties of protoplanetary
discs, we have combined the capabilities of the radiative transfer code MCFOST
with the gas thermal balance and chemistry code ProDiMo to compute a grid of
300 000 disc models (DENT). We present a comparison of the first Herschel/GASPS
line and continuum data with the predictions from the DENT grid of models. Our
objective is to test some of the main trends already identified in the DENT
grid, as well as to define better empirical diagnostics to estimate the total
gas mass of protoplanetary discs. Photospheric UV radiation appears to be the
dominant gas-heating mechanism for Herbig stars, whereas UV excess and/or
X-rays emission dominates for T Tauri stars. The DENT grid reveals the
complexity in the analysis of far-IR lines and the difficulty to invert these
observations into physical quantities. The combination of Herschel line
observations with continuum data and/or with rotational lines in the
(sub-)millimetre regime, in particular CO lines, is required for a detailed
characterisation of the physical and chemical properties of circumstellar
discs.
