Gateway of Metal

When Gateway of Metal enters the battlefield, each opponent puts a 0/1 Critter artifact creature token onto the battlefield tapped.
Whenever an artifact creature with converted mana cost 3 or less attacks, its controller may add one mana of any color to their mana pool.
On your end step, if you attacked with an artifact with converted mana cost 3 or less, flip a coin. If the result is heads, each opponent loses 1 life. Otherwise, you lose 3 life and each opponent gains 1 life.