Most of our current knowledge of protoplanetary disks originates from studies of the dust component (e.g. spectral energy distributions (SED), scattered light images, emission maps) despite the fact that dust comprises only 1% of the initial mass of the disk.In contrast, the dominant and essential gas component of a protoplanetary disk has proven more difficult to observe thus far. Combining the gas and dust information from protoplanetary disks is particularly important for understanding disk evolution. The Herschel Space Observatory, due to be launched in April of this year, will open an unexplored wavelength window in the far infrared regime, providing access to high-quality observations of the gas in disks. I have been invited to join the "GAS in Protoplanetary Systems" (GASPS) Open Time Key Project that will observe both continuum (dust) and line (gas), namely [CII], [OI] and water, emission for an unbiased sample of 240 young stars, spanning a large range of stellar masses as well as the entire duration of planet formation. This DiskEvol program will tackle the complex problem of combining consistently the constraints on the gas phase of a disk, provided by the Herschel observations, with our existing studies of the dust phase. Interpretation of gas observations is complicated by the large number of physical processes at play: chemistry, excitation and destruction of molecules, freeze-out onto the dust grains, to name a few. But, this is of particular importance as the dissipation of abundant gas remnant from star formation limits the timescale for giant planet formation, controls the dynamics of planetary bodies (of all sizes) during their formation and determines the final architecture of the planetary system. This proposal will rely on 1) the preparation of a database including observations and models for the GASPS project, resultant from ancillary data for GASPS in the millimetre regime and the generation of grids of radiative transfer models (SEDs and line fluxes), respectively. This initial work will allow 2) a detailed analysis of the GASPS program through a statistical comparison of the GASPS observations with the predictions from the grid of models. This global study, of the large sample of disks observed by GASPS, will be 3) extended and completed through finer detailed modelling of a selected sample of representative sources for which we will obtain a complete view of the dust structure and gas chemistry using simultaneous interpretation of continuum observations, resolved emission maps in low-level rotational lines of CO, in addition to follow-up observations with HIFI, a high-spectral resolution instrument on-board Herschel. DiskEvol will provide an unprecedented inventory of gas and dust in protoplanetary disks, transforming our understanding of disk evolution by addressing key questions on the timescales and main mechanisms of dust and gas evolution within disks. In addition, the long-lasting value of DiskEvol results is of exceptional importance in the era of ALMA and JWST.