Hi
As you may know, the GRI Guidelines have been existing for more than 10 years now; it started as a framework for environmental reporting and later transformed into a framework for Sustainability reporting. ISO 26000 was issued only last year and the subject matter dealt with in both, one a guideline on Corporate Responsibility and the other a guideline for reporting Organizational Responsibility (Sustainability) performance, is the same. Both have evolved (and continue to evolve) through multi-stakeholder dialogue. Hence there is almost 100 % congruence as regards the subject matter addressed in these two documents. Of course, one can always point out gaps in either of them. Under these circumstances, my inference is that each is complementary to the other; at the same time they have different roles to play in the development of the subject.
With kind regards,
Ramakrishnan