Built in the Indo-saracenic style, the Gateway of India is meant to commemorate the visit of King George V and Queen Mary to Bombay, prior to the Darbar in Delhi in December 1911. The foundation stone was laid on March 31, 1911 and George Wittet's final design sanctioned in August 1914. Between 1915 and 1919 work proceeded on reclamations at Apollo Pier for the land on which the gateway and the new sea wall would be built. The foundations were completed in 1920.
The Gateway is built from yellow Kharodi basalt and reinforced concrete. The central dome is 48 feet in diameter and 83 feet above ground at its highest point. The whole harbour front was realigned in order to come in line with a planned esplanade which would sweep down to the centre of the town. The cost of the construction was Rs. 21 lakhs, borne mainly by the Government of India. For lack of funds, the approach road was never built, and the Gateway now stands at an angle to the road leading up to it.
The construction was completed in 1924, and the Gateway opened on December 4, 1924 by the Viceroy, Earl of Reading.
Ironically, the last British troops to leave India, the First Battalion of the Somerset Light Infantry, passed through the gate in a ceremony on February 28, 1948.
Above info sourced from http://theory.tifr.res.in/bombay/architecture/building/gateway.html