  A relativistic radio component, which moves in a direction close to the sky
plane, will increase in flux density when it decelerates. This effect is the
basis for the qualitative model for GPS galaxies we present in this paper,
which can explain their low-variability convex spectrum, their compact double
or compact symmetric morphology, and the lack of GPS quasars at similar
redshifts. Components are expelled from the nucleus at relativistic speeds at a
large angle to the line of sight, and are decelerated (eg. by ram-pressure or
entrainment of the external gas) before contributing to a mini-lobe. The young
components are Doppler boosted in the direction of motion but appear fainter
for the observer. The non-relativistic mini-lobes dominate the structure and
are responsible for the low variability in flux density and the convex radio
spectrum as well as the compact double angular morphology. Had the same source
been orientated at a small angle to the line of sight, the young components
would be boosted in the observer's direction resulting in a flat and variable
radio spectrum at high frequencies. Hence the characteristic convex spectrum of
a GPS source would not be seen. These sources at small angles to the line of
sight are probably identified with quasars, and are not recognized as GPS
sources, but are embedded in the large population of flat spectrum variable
quasars and BL Lac objects. This leads to a deficiency in GPS/CSOs identified
with quasars.
