The Gate of the Sun God is a special building released for the Guild Expeditions. It can be obtained as a reward for a successful Guild Expedition encounter or found in a relic.
It is one of two unique decorations released exclusively for GE. The Gate of the Sun God provides happiness. When polished, its happiness is doubled for 12 hours.
Together with Face of the Ancient it was one the best happiness-providing buildings available in the game until update 1.95, providing much more happiness per tile than premium decorations and even premium cultural buildings. Update 1.95 reduced the happiness value to only 56% of the original, thus making it only a bit better than regular decorations and worse than premium ones.
It can be used as a quick, free boost to happiness for quests requiring increased happiness.