  It is well known that --differing from ordinary gauge systems-- canonical
gauges are not admissible in the path integral for parametrized systems. This
is the case for the relativistic particle and gravitation. However, a time
dependent canonical transformation can turn a parametrized system into an
ordinary gauge system. It is shown how to build a canonical transformation such
that the fixation of the new coordinates is equivalent to the fixation of the
original ones; this aim can be achieved only if the Hamiltonian constraint
allows for an intrinsic global time. Thus the resulting action, describing an
ordinary gauge system and allowing for canonical gauges, can be used in the
path integral for the quantum propagator associated with the original
variables.
