The color identity of a gate depends on the color(s) of mana it can tap for, as represented by the symbols on the card
In this case, all five are cheap, common-rarity creatures with an ability tied to one of Tarkirs three-color clans, and another that allows you to turn mana into a color from that clan
Deck Overview: Focuses heavily on playing Enchantments, both as Auras to buff ones Creatures as well as those that stay on the Battlefield and provide synergistic effects to create an unstoppable, enchanted army of death and destruction
Plus, some decks just want critical mass of these effects
While the initial batch of Universes Beyond releases were Secret Lair drops, the upcoming ones are Commander decks - such as Warhammer 40,000 - or full sets like The Lord of the Rings