Hinduism...Why does Krishna say in Bhagavad Gita that among the mantras......?

He is Gayatri. He gives top status to Gayatri Mantra amongst the mantras. Why does he not say that the Hare Krishna Mahamantra is the best or 'Om namoh narayana' mantra is the best? If he was an avatar of Vishnu or even if he was the Supreme God, he could have stressed the importance of chanting the name of Krishna or Vishnu than chant a mantra to the Sun Deity. Your inputs are welcome..thanks.