Gaia is a project of European Space Agency. Gaia is a satellite which should measure 3-dimensional positions of about one billion stars, quasars and solar system objects as well as 3-dimensional velocities of those objects and their physical properties (through multi-band photometry and spectrometry of each source). The accuracy of measurements which will be produced by Gaia is about 1 microarcsecond. This corresponds to the thickness of a standard sheet of paper (50 micrometer) as seen from a distance equal to the Earth diameter. A group in Lohrmann observatory actively participates in this Project. Our main fields of expertise relevant for Gaia are relativistic modelling, testing relativity and solar system science. Lohrmann Observatory coordinates a Gaia collaboration REMAT responsible for relativistic modelling of Gaia data and for the use of microarcsecond astrometric observations for tests of relativity and other aspects of fundamental physics. We also participate in all aspects of astrometric data processing for Gaia.