GSAS-II is able to import phase information, single-crystal structure factors and powder diffraction data from CIFs. This is done from the menus associated with data imports (as discussed in 15. CIFs are parsed using the Python pyCifRW package from James Hester. James also leads the IUCr committee that oversees CIF development, so it may come as no surprise that pyCifRW requires that a CIF have correct syntax in order to be read. Databases do contain non-compliant CIFs and there is some software is reasonably tolerant of syntax errors. Should you encounter a CIF that GSAS-II will not read, you are recommended to use the free CCDC enCIFer program (https://www.ccdc.cam.ac.uk/solutions/software/encifer/), which can help you locate and fix CIF syntax errors. There are several other programs that can do this, but I am not familiar with any. You might also have success with asking an AI agent, such as CoPilot or Gemini to examine and correct the file for syntax errors.
One special type of CIF processing takes place when files created on the ISODISTORT web site are imported, as will be discussed in ยง28.3.4. These contain special CIF data items that describe how atomic parameters are set from representational analysis modes.
While reading of CIFs is a fairly complex task, as there is considerable complexity in how data may be organized in them, the importers function largely without any need for additional input, provided that the CIF has been properly written. One provides a file name, potentially selects which block(s) contain the desired data, when a choice is needed, and GSAS-II will then have the ingested the phase or histogram.