GSAS-II is hosted on GitHub.com and is typically downloaded from there. Links to information provided about GSAS-II is provided on the GitHub home page, https://GSASII.github.io. For installation instructions, please see the installation section of that website (https://advancedphotonsource.github.io/GSAS-II-tutorials/install.html) . Most people will choose to download one of the GSAS2MAIN self-installers, but there are many different ways that GSAS-II can be installed depending on your own preferences and goals. The installation instructions web pages provide a detailed discussion of the options and advantages.
The source code for GSAS-II is found in GitHub repository https://github.com/AdvancedPhotonSource/GSAS-II. It is worth noting that the source code for GSAS-II is installed as part of the GSAS-II installation, so we try to keep the size of that repo small. Thus, there are three other repositories associated with GSAS-II with material that is less frequently downloaded. The GSAS-II documentation, including tutorial web pages, and input used to create the web pages, help pages and newer tutorials is found at https://github.com/AdvancedPhotonSource/GSAS-II-tutorials. Routines used to compile and package GSAS-II, as well as the distribution files for installation are found in repo https://github.com/AdvancedPhotonSource/GSAS-II-buildtools and finally the files used to create this book are found at https://github.com/briantoby/PowderCrystallography. There is also a GSAS-II code developer’s manual, which is a formatted version of text and comments found in the source code. This is hosted on a separate site, https://gsas-ii.readthedocs.io/. The documentation section of the GSAS-II home page provides access to the manuals in several formats.