  We use planetary nebulae (PNe) as probes to determine the Galactic radial
oxygen gradients, and other abundance patterns. We select data homogeneously
from recent data sets, including PNe at large Galactocentric distances. The
radial oxygen gradient calculated for the general PN population, which probes
the region between the Galactic center and out to $\sim$28 kpc, is shallow,
with slope $\sim$-0.02 dex kpc$^{-1}$, in agreement with previous findings. We
looked for time evolution of the metallicity gradient using PNe with different
age progenitors as metallicity probes. We identify PNe whose progenitor stars
are younger than 1 Gyr (YPPNe), and those whose progenitor stars are older than
7.5 Gyr (OPPNe), based on the comparison between evolutionary yields and
elemental abundances of the PNe. By studying OPPNe and YPPNe separately we
found that: (i) The OPPNe oxygen gradient is shallower ($\sim-0.015$ dex
kpc$^{-1}$) than that derived from YPPNe ($\sim-0.027$ dex kpc$^{-1}$); (ii)
the OPPNe inner radial distribution of oxygen is compatible with no gradient to
the radial extent of the thick disk population ($\sim$10 kpc), similarly to
what has been observed in thick disk stars; (iii) planetary nebulae (especially
OPPNe) indicate that significant gradient slope is limited to Galactocentric
distances between $\sim$10 to $\sim$13.5 kpc, as observed for open clusters and
field stars. Outside this range, the distribution is almost flat. We found that
the radial oxygen gradient is steeper for a PN population closer to the
Galactic disk, similarly to what is observed in the general stellar population
by the SEGUE survey. We use our novel population dating to compare our results
with current chemical evolutionary models, and with gradients from other
Galactic populations, for insight on galaxy chemical evolution.
