A system for electron cyclotron resonance plasma heating (ECRH) has been recently installed at the GDT (Gas Dynamic Trap) facility at Budker Institute. The system is based on two 5.5-mm gyrotrons and is designed to deliver two microwave beams with total power of 700 kW and X-mode polarization that are absorbed at the fundamental cyclotron harmonic. A significant increase of basic plasma parameters (energy content, electron temperature, neutron flux) during the injection of microwave radiation has been registered. In particular, the on-axis electron temperature was increased from 200 eV to 600 eV in several shots with ECRH, which establishes a new record for this class of magnetic installation.