  GRAVITY is a new instrument to coherently combine the light of the European
Southern Observatory Very Large Telescope Interferometer to form a telescope
with an equivalent 130 m diameter angular resolution and a collecting area of
200 m$^2$. The instrument comprises fiber fed integrated optics beam
combination, high resolution spectroscopy, built-in beam analysis and control,
near-infrared wavefront sensing, phase-tracking, dual beam operation and laser
metrology [...]. This article gives an overview of GRAVITY and reports on the
performance and the first astronomical observations during commissioning in
2015/16. We demonstrate phase tracking on stars as faint as m$_K$ ~ 10 mag,
phase-referenced interferometry of objects fainter than m$_K$ ~ 15 mag with a
limiting magnitude of m$_K$ ~ 17 mag, minute long coherent integrations, a
visibility accuracy of better than 0.25 %, and spectro-differential phase and
closure phase accuracy better than 0.5{\deg}, corresponding to a differential
astrometric precision of better than 10 microarcseconds ({\mu}as). The
dual-beam astrometry, measuring the phase difference of two objects with laser
metrology, is still under commissioning. First observations show residuals as
low as 50 {\mu}as when following objects over several months. We illustrate the
instrument performance with the observations of archetypical objects for the
different instrument modes. Examples include the Galactic Center supermassive
black hole and its fast orbiting star S2 for phase referenced dual beam
observations and infrared wavefront sensing, the High Mass X-Ray Binary BP Cru
and the Active Galactic Nucleus of PDS 456 for few {\mu}as spectro-differential
astrometry, the T Tauri star S CrA for a spectro-differential visibility
analysis, {\xi} Tel and 24 Cap for high accuracy visibility observations, and
{\eta} Car for interferometric imaging with GRAVITY.
