The worship of goddess Gayatri is relatively new. I am not sure when these temples were built because in ancient scriptures you here so much more about Durga, Kali, Parvati, Lakshmi, Saraswati. The goddess Gayatri became well known mainly after the efforts of Sri Ram Sharma Acharya ji in the previous century. Number of temples have come up after that.
The Maharaja of Mysore was a Shakta, a Kshatriya devoted to power and not so much to wisdom. He naturally worshipped Durga and the festival of goddess Durga is celebrated there even today with much pomp and show.
Affectionately,
Alok Da
