Does a character die equipped with Magic Helmet, Greater Gear, stop all damage at them when they block a character die with overcrush since that equipped die is not effected by a characters ability or an action die?
"Equip (attach to a character with Can Equip Gear). This equipped character cannot be affected by your opponent's action dice or character abilities."
No, Magic Helmet does not prevent the application of damage to a player due to Overcrush. This is because the Overcrush ability affects the player, not the character with Magic Helmet equipped.
The relevant text from the rulebook is as follows:
DC Dice Masters: Justice League Rulebook Page 26:
Overcrush: When attacking, if this character knocks out all of its blockers, it deals any leftover damage to your opponent.
-The Dice Masters Rules Team