  Context. The xGASS and xCOLD GASS surveys have measured the atomic (HI) and
molecular gas (H2) content of a large and representative sample of nearby
galaxies (redshift range of 0.01 $\lt$ z $\lt$ 0.05). Aims. We present optical
longslit spectra for a subset of the xGASS and xCOLD GASS galaxies to
investigate the correlation between radial metallicity profiles and cold gas
content. In addition to data from Moran et al. (2012), this paper presents new
optical spectra for 27 galaxies in the stellar mass range of 9.0 $\leq$ log
Mstar/Msun $\leq$ 10.0. Methods. The longslit spectra were taken along the
major axis of the galaxies, allowing us to obtain radial profiles of the
gas-phase oxygen abundance (12 + log(O/H)). The slope of a linear fit to these
radial profiles is defined as the metallicity gradient. We investigated
correlations between these gradients and global galaxy properties, such as star
formation activity and gas content. In addition, we examined the correlation of
local metallicity measurements and the global HI mass fraction. Results. We
obtained two main results: (i) the local metallicity is correlated with the
global HI mass fraction, which is in good agreement with previous results. A
simple toy model suggests that this correlation points towards a 'local gas
regulator model'; (ii) the primary driver of metallicity gradients appears to
be stellar mass surface density (as a proxy for morphology). Conclusions. This
work comprises one of the few systematic observational studies of the influence
of the cold gas on the chemical evolution of star-forming galaxies, as
considered via metallicity gradients and local measurements of the gas-phase
oxygen abundance. Our results suggest that local density and local HI mass
fraction are drivers of chemical evolution and the gas-phase metallicity.
