Abstract　 　 
The high-mass end of the galaxy population is important to probe in order to understand mass assembly in the universe. For this purpose we utilize the long-slit spectroscopy from Moran et al. (2012) obtained for a sub-sample of ~230 galaxies of the GALEX Arecibo SDSS Survey (GASS). This survey consists (in its final stage) of ~1000 massive galaxies (M > 10^10 Msun) selected without any bias to morphology. The GASS galaxies have measured gas mass fractions for both neutral Hydrogen (full sample) and molecular Hydrogen (~350 galaxies). This allows us to investigate and link gas and stellar properties as well as gas and stellar kinematics. Hence we cover an extensive parameter space useful for studying the evolution of a complete sample of low-redshift massive galaxies. Using the long-slit spectroscopy we perform a detailed study of the stellar population gradients of the galaxies in GASS. Ages, metallicities and the element abundance ratios [C/Fe], [N/Fe], [O/Fe], [Mg/Fe], [Ca/Fe] and [Ti/Fe] are derived through the fitting of modelled absorption lines, using the technique from Johansson, Thomas and Maraston (2012). This is based on the stellar population models of absorption lines from Thomas, Maraston and Johansson (2011). We present results for the relations between stellar population gradients and gas mass fractions, gas-phase metallicities, star formation rates, galaxy morphologies etc. We find a particularly strong relation between molecular gas fraction and age of the stellar populations, where the latter continuously increase with decreasing gas fraction. We further find that the strength of the stellar population gradients depend on gas fraction. These results put strong constraints on galaxy formation, in terms of the relation between star formation shut down and stellar accretion.