They are publicly available and the exams are held throughout the country (UK). You send in your fee and you go and hopefully demonstrate that you are of a standard set for that grade. I have no idea if they are available outside of the UK. I suspect they aren't.
Edit: They are available...
http://www.registryofguitartutors.com/regions/usa.htm
I don't know if there is a curriculum published on the internet, but the books that list the requirements are available from the relevant bodies.
http://www.registryofguitartutors.com/
I notice, by the way, that grades 6-8 can count towards UCAS points (official college/university requirements in the UK), but I guess it will be limited to certain courses.