Qubes OS is a compilation of software packages, each under its own license. The compilation is made available under the GNU General Public License version 2 (GPLv2). However, the license for this compilation does not supersede the license of any package included in the compilation.
The source code of Qubes OS is contained in repositories under the @QubesOS account on GitHub. This source code is made available under GPLv2, unless there is a LICENSE file in the root of the containing repository that specifies a different license.
The full text of the GPLv2 license can be found here.