GREGOR is the project of a high-resolution solar telescope with an aperture of 1.5m and an effective focal length of about 55m. It is designed to support ground-based accurate, high sensitive spectro-polarimetry at visible and IR wavelengths in the solar photosphere and chromosphere for studying the dynamics of the solar atmosphere and the underlying physical processes. The concept of polarimetric measurements with GREGOR is based on several unique and highly specialized post-focus polarimeters like POLIS or TIP and a polarimetric equipment (GPU) situated near the telescope's secondary focus F2 where the optical properties are still rotationally symmetric and the telescope can be regarded as polarization free at the 10-4 level. The GPU is an integral part of the telescope, consisting of a calibration unit and a modulation unit. The calibration unit allows to calibrate the modulation unit as well as polarimeters built in any post-focal device. It consists of a linear polarizer and quarter wave retarders for the visible and the IR spectral range and is located in front of the modulation unit. The modulation unit is supposed to permit a very efficient polarimetric analysis in the spectral range from 400 nm to 700 nm. It consists of two electro-optical modulators and a prism as linear polarizer.