  After a short review of experimental foundations of metric theories of
gravity, the choice of general relativity as a theory to be used for the
routine modeling of Gaia observations is justified. General principles of
relativistic modeling of astronomical observations are then sketched and
compared to the corresponding Newtonian principles. The fundamental reference
system -- Barycentric Celestial Reference System, which has been chosen to be
the relativistic reference system underlying the future Gaia reference frame is
presented. Principal relativistic effects in each constituent of a relativistic
model of astronomical observations are briefly elucidated. The structure of a
relativistic model of positional observations which can be used as a standard
relativistic model for Gaia is sketched. The physical meaning of the Gaia
reference frame is discussed. It is discussed also how Gaia observations can be
used to verify general relativity.
