"Of the four fundamental forces in nature, gravity is the least tested one. A uniquely accessible laboratory for gravitational physics is the Galactic Center, hosting the closest massive black hole with 4 million solar masses. Its vicinity and the advent of high-resolution, near-infrared techniques allowed for the observation of ≈ 30 individual stellar orbits in its gravitational field. The stars are ideal test particles for the potential in which they move, and thus the keys to directly probing the gravitational potential are ultra-precise astrometry and radial velocities of the stars.All data to date can be described by Keplerian orbits around a single central mass. However, relativistic effects will lead to post-Newtonian orbits. These effects will soon be detectable with near-infrared interferometry for faint objects. With the next-generation instrument GRAVITY for the Very Large Telescope Interferometer of ESO the achievable resolution is ≈ 4 mas, and the astrometric accuracy ≈ 10 µas - the scale of the event horizon. This is an improvement of a factor ≈ 20 compared to current, adaptive-optics based data.PROGRESO aims at detecting the relativistic effects in the stellar orbits around Sgr A*. Also, the emission from hot spots close to the last stable orbit might be used. Scientifically, this will test gravity in a so far unprobed regime of mass and space-time curvature. The spin of the black hole might be determined in this way, and in the most optimistic case test the 'no-hair' theorem. With PROGRESO I would like to start a research group that focuses on interpreting the novel interferometric data of the Galactic Center with yet to be determined instrumental systematics, on exploiting it scientifically and on optimally combining it with existing and future data of the present instruments.The proposed research group will be hosted at MPE (Garching, Germany), the PI institute for GRAVITY, and therefore have access to commissioning and guaranteed time data."