My theory is Sunbreaker has 50% DR but when using gear with the solar based subclass armor perk, has 55% DR (making you able to withstand 1 GG shot). If that theory holds true, a stormcaller and blade dancer (with max armor builds) might be able to do the same with said perk on gear but for arc. I don't think Sunbreaker by itself withstands 1 GG shot is the point. The class focused on armor with a perk from another piece of armor does.