The Elemental Gate is the summon building where you use Summoning Spheres 
to perform your hero summons.
This building is available to you after reaching Castle level 6.
Unlike the Summon Gate, the Elemental Gate is different because you are able to choose which Element of hero you wish to summon. In addition, the that you use give the best odds for high-level heroes in the game:
[odds here]
Odds for summoning heroes with this item also include the chance for 5★ hero shards, as well as Magic Dust 
that you can use at the Laboratory.
You can maximize your summons when you do them during the Elemental Summons event - considered one of the best events in Puzzle Breakers, the Elemental Summons event offers you strong heroes as a reward for performing summons for each Element.