The Return to Ravnica set introduces a new land subtype: Gate. Each guild has a Gate card with Guildgate in its name that taps for mana of either of that guild’s colors.
Land — Gate
Azorius Guildgate enters the battlefield tapped.
Tap: Add White Mana or Blue Mana to your mana pool.
The subtype Gate has no special rules significance, but other spells and abilities may refer to it.
Gate is not a basic land type.