GECAM (Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor) is a constellation of two small X-ray and gamma-ray all-sky observatories to search for gamma-ray counterparts to gravitational wave events. Each satellite features a dome-shaped array of 25 Gamma-ray detectors (GRD) and 8 Charged particle detectors (CPD). Together the satellites will provide a FOV of 100% all-sky. The sensitivity of the detectors is ~2E-8 erg/cm2/s. They will provide a localization of ~1°. The detectors are sensible in the energy band from 6 keV to 5 MeV. Besides detecting the radiation from gravitatinal wave events, the satellites will also be contributing in detectiong Ultra-long GRBs, X-ray Flashes, X-ray-rich GRBs, Magnetars and Terrestrial Gamma-ray Flashes.