The Guru Granth Sahib reached its present form in two stages, and both are visible in the volume.
The first was the compilation of 1604 under Guru Arjan, who gathered the compositions of his four predecessors and his own, together with the work of the bhagats, and fixed the arrangement by rag that the volume still follows. That act closed the collection against material circulating under the Gurus' names that the compilers did not accept, which is as much a part of what a canon is as the inclusion.
The second was the recension attributed to Guru Gobind Singh, in which the compositions of the ninth Guru, Tegh Bahadur, were incorporated. They were not appended. They were inserted at their proper place within each rag section, in the order of the succession, exactly where the existing scheme required them to go, and where the measure they were composed in had no section, a new heading was created at the end of the sequence.
The tenth Guru added none of his own compositions to the volume, which is a fact of the first importance. He ended the line of human Gurus and invested the scripture with that authority in 1708, and the volume he invested contains the work of nine Gurus and not ten.
After that the canon is closed in the strongest possible sense: nothing can be added to a Guru, and the volume has not changed since.