The galop is usually created by playing triplets. Altho I can't play them as fast has some of the Iced Earth songs, the basic principle is still the same. The further away from the string you end up after striking a string, the longer it takes to hit it again thus the slower you'll be able to play. Triplets with as little motion as possible. You really want to control your picking hand so it doesn't go too far from the string.
Start very slowly and concentrate on reducing the distance of your pick from the strings. Then work on gaining speed while keeping that limited distance.
Most of the fast pickers I seen uses heavy picks.