I just found out that not only does GM Shield halves elemental projectile attacks (fire, air, water, etc colored projectiles), but "Of Shielding" and "Of the Storm" enchantments as well!
(Those were supposed to only half physical ranged attacks: arrows)
GM Shield rarely shows up in a normal game (since only the knight can learn it but my knights never bother with shields) , but needless to say, using Of Shielding or Of the Storm enchantments will make the game MUCH easier for the character wearing it!
It says it acts like the Shield spell, but that spell does nothing, in either MM7 or MM8.
That explains why some of my characters never took as much damage as the ohers even though they had the same resistance or armor skills: they had an item with the Of Shielding property.