Crytek has made CryEngine an open source game development engine for anyone to use, and you can get started by downloading all necessary materials from Github. Users are free to use CryEngine, which was recently updated to its 5.1 version, to create games and release them as commercial products as long as they do not violate Crytek’s terms of service.
These rules include not using CryEngine to create software that could be considered as a military project, gambling, science based, architectural based, pornography, simulation (technical, scientific, other) or a serious game. That last term means games that aren’t created for the sole purpose to entertain but for any of the other above terms.
The full terms and conditions for CryEngine can be read here.