All POSIX operating systems (Unix, Linux, MacOSX etc) are supported. Microsoft Windows is not supported. The current version requires Java 1.8. Oracle Java and OpenJDK are both officially supported.
All forms of GATK4 are fully open-source under a BSD 3-clause license. Please see the Licensing page for more information.
The source code for the full GATK package is available in the broadinstitute/gatk repository on Github.