  We use an extended and homogeneous data set of Galactic planetary nebulae
(PNe) to study the metallicity gradients and the Galactic structure and
evolution. The most up-to-date abundances, distances (calibrated with
Magellanic Cloud PNe) have been employed, together with a novel homogeneous
morphological classification, to characterize the different PN populations. We
confirm that morphological classes have a strong correlation with PN Peimbert's
Type, and also with their distribution on the Galactic landscape. We studied
the alpha-element distribution within the Galactic disk, and found that the
best selected disk population, together with the most reliable PN distance
scale yields to a radial oxygen gradient of d[log(O/H)]/dR=-0.023 +- 0.006 dex/
kpc for the whole disk sample, and of d[log(O/H)]/dR= -0.035+-0.024,
-0.023+-0.005, and -0.011+-0.013 dex/kpc respectively for Type I, II, and III
PNe. Neon gradients for the same PN types confirm the trend. Accurate
statistical analysis show moderately high uncertainties in the slopes, but also
confirm the trend of steeper gradient for PNe with more massive progenitors,
indicating a possible steepening with time of the Galactic disk metallicity
gradient. The PN metallicity gradients presented here are consistent with the
local metallicity distribution; furthermore, oxygen gradients determined with
young and intermediate age PNe show good consistency with oxygen gradients
derived respectively from other young (OB stars, HII regions) and intermediate
(open cluster) Galactic populations. We also extend the Galactic metallicity
gradient comparison by revisiting the open cluster [Fe/H] data from high
resolution spectroscopy. The analysis suggests that they could be compliant
with the same general picture of a steepening of gradient with time.
