The Summon Gate is the main building where you conduct hero summons, using Basic Summoning Scrolls 
or Royal Summoning Scrolls 
.
Available from the very start of the game, you should frequent the Summon Gate every day, since conducting hero summons is a Daily Mission for both and .
[Summoning odds here][Summoning odds here]
One thing to remember: when conducting a hero summon from the Summon Gate, you will always summon a hero straight to your hero inventory.
It can be useful to know that with every summon you perform here, a hero will always take a space...especially if you plan on doing some mass summons to complete the Royal Summons event. Having some 4★ heroes at the ready for Hero Fusions could help you save space in this regard.