Hey, I was just accepted into Berklee last Saturday with my GED. I will be a vocal principle. I guess for any advice, I'd say just tell them how important learning is to you, and how passionate you are about music and Berklee. Of course you need to have a level of talent and skill, but having your GED doesn't dismiss you from getting in. Look at me.
Just promote your passion and desire for Berklee and you may also want to explain why exactly you got your GED in you essay. At least I did. As long as they can see that you got your GED for a good a reason and that you didn't get it just because you were lazy and didn't want to finish high school then you should be fine. Also getting really high scores on your GED STRONGLY helps. My scores were immaculate and that's very important, especially if your high school grades were bad.
Hopes this helps. If you have any questions just let me know.