I just applied for my DS7 in February and he got accepted to the program. I think the most important thing is the test score. The answers to the questions don't really need to be long. I've used bullet points for a few questions and gave examples in other questions. I think the documented examples can be submitted as supplemental support for the application. I asked DSs' piano teacher to be the nominator as he has known my son for more than 2 years and DS has shown him his other abilities other than playing piano. I think the GT teacher would be a good nominator if he/she knows your son well.

HTH. Good luck!