Iron Grip and Iron Will

The difference between this two gems is that Iron Will increase spell damage and Iron Grip adds physical damage not to a spell but to a weapon?

will spell =/= attack grip

Iron Will adds a bonus to spelldamage based on Strength. 5 Strength = 1% Increased Spell Damage for the linked spells.

Iron Grip adds a bonus to Physical Projectile Attack Damage based on Strength. 5 Strength = 1% Increased Physical Projectile Attack Damage. This exists as a Keystone Passive and a Support Gem. The gem will only affect the linked skills.

These mimic the intrinsic Strength bonus to Melee Physical Attack Damage.

*Edited to clarify Iron Grip tags.

Right, iron will adds the same type of bonus as regular 'increased spell damage' nodes.

Iron grip essentially works as 'inc bow physical damage' or 'inc wand physical damage' nodes, it's different then 'inc projectile damage' nodes that work on all types of damage, even on projectile spells. See, it doesn't specify, it just says 'projectile damage', this is a very literal game and it's inconsistent only occasionally.