Sorath is a large and heavily worked section, with the fourth and fifth Gurus strongly present and a substantial body of work by the bhagats, and it carries a var. Its name is territorial rather than descriptive: Sorath is a region of the Kathiawar peninsula in western India, and the measure carries the name of the place its tradition associates it with.
That is a reminder worth holding on to. The musical vocabulary the Gurus used was not local and it was not new. These measures had been carried across the subcontinent for centuries by musicians, by pilgrims and by the movement of courts, and by the sixteenth century a rag named for Gujarat, a rag named for the Punjab and a rag named for the eastern country could all sit in one repertoire without anyone finding it remarkable.
The Granth inherits that whole geography. A volume compiled in the Punjab, in a script devised for Punjabi, in a composite register with Persian and Braj and Sanskritic vocabulary running through it, sets its hymns to measures named after regions its authors never visited. Nothing about it is parochial, and the compilers made no attempt to make the musical scheme more local than it was.
For a reader tracing the movement of forms across pre-modern India, the rag headings of the Guru Granth Sahib are a substantial and precisely dated body of evidence about what was in use, where, and by whom.