What exactly do they do? And does anyone know what skills are important for them??

I found on wikipedia:

In American football, a gunner (also often referred to as a "shooter" is a player on kickoffs and punts who specializes in running down the sideline very quickly in an attempt to tackle the kick returner or the punt returner. Gunners must have several techniques in order to break away or "shed" blockers, and have good agility in order to change their running direction quickly. Gunners on the punt team also must be able to block or catch.