It's almost certainly not the GRS (because of the way the scores are grouped in the rubric), but it could be one of the other common rating scales.
Rating scales for giftedness do have flaws, as you can tell if you've ever read the manual of any one of them, in which they rip all the other scales, but they are still better than teacher nomination.
Another possibility is that the student performance criteria are a set of activities that students will be asked to do at a later stage in the process. One of my old districts had a performance assessment, in which nominees were invited to participate in a day at the self-contained GT building, where they engaged in group and individual semi-structured activities while being observed and rated by the teachers of the gifted. Which did have the advantage that, whether or not they were accurately selecting for the most gifted students, they were definitely selecting for the ones most likely to be successful in the program.
