The code for SCP13 is licensed under the GNU Affero General Public License v3, which can be found in full in LICENSE-AGPL3.txt.
Code with a git authorship date prior to 1420675200 +0000 (2015/01/08 00:00) is licensed under the GNU General Public License version 3, which can be found in full in LICENSE-GPL3.txt.
All code where the authorship dates are not prior to 1420675200 +0000 is assumed to be licensed under AGPL v3, if you wish to license under GPL v3 please make this clear in the commit message and any added files.
If you wish to develop and host this codebase in a closed source manner you may use all commits prior to 1420675200 +0000, which are licensed under GPL v3. The major change here is that if you host a server using any code licensed under AGPLv3 you are required to provide full source code for your servers users as well including addons and modifications you have made.
See here for more information.
tgui clientside is licensed as a subproject under the MIT license. Font Awesome font files, used by tgui, are licensed under the SIL Open Font License v1.1 tgui assets are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
See tgui/LICENSE.md for the MIT license. See tgui/assets/fonts/SIL-OFL-1.1-LICENSE.md for the SIL Open Font License.
All assets including icons and sound are under a Creative Commons 3.0 BY-SA license unless otherwise indicated.