  Gaia Data Release 3 provides novel flux-calibrated low-resolution
spectrophotometry for about 220 million sources in the wavelength range 330nm -
1050nm (XP spectra). Synthetic photometry directly tied to a flux in physical
units can be obtained from these spectra for any passband fully enclosed in
this wavelength range. We describe how synthetic photometry can be obtained
from XP spectra, illustrating the performance that can be achieved under a
range of different conditions - for example passband width and wavelength range
- as well as the limits and the problems affecting it. Existing top-quality
photometry can be reproduced within a few per cent over a wide range of
magnitudes and colour, for wide and medium bands, and with up to millimag
accuracy when synthetic photometry is standardised with respect to these
external sources. Some examples of potential scientific application are
presented, including the detection of multiple populations in globular
clusters, the estimation of metallicity extended to the very metal-poor regime,
and the classification of white dwarfs. A catalogue providing standardised
photometry for ~220 million sources in several wide bands of widely used
photometric systems is provided (Gaia Synthetic Photometry Catalogue; GSPC) as
well as a catalogue of $\simeq 10^5$ white dwarfs with DA/non-DA classification
obtained with a Random Forest algorithm (Gaia Synthetic Photometry Catalogue
for White Dwarfs; GSPC-WD).
