It's unclear where GRI-checked reports fall under the credit requirements - would they be considered self-declared for 1/2 value or third-party verified for full value? My guess was self-declared, but perhaps there could be an in-between (3/4 value?) for GRI-checked reports. Also, for those who haven't used it before, here's a link to the GRI Sustainability Disclosure Database: http://database.globalreporting.org/search
I agree with Christine above; the GRI Sustainability Disclosure Database is very helpful in documenting this credit.
Another good source is the UN Global Compact database: https://www.unglobalcompact.org/what-is-gc/participants