I am new in ISKCON. I want to know about deity left to Lord Krishna (Gaur Nimai), so that I can worship Them sincerely.
It is very nice to hear from you that you are interested to know about Them. I would to like to make few corrections.
Deities right to Krishna are Sri Sri Gaur Nitai and not Gaur Nimai. Gaur means Lord Gauranga Mahaprabhu. Nimai is another name for Lord Gauranga Mahaprabhu. Also usually They are kept to the right to Krishna and not to left.
Gaura Nitai are merciful incarnations for the age of Kali. They appeared almost 500 years before in the district of Nadia, in West Bengal. They may appear similar. Amongst Them, toward our left is Lord Nityananda, who is none other than Sri Balaramji. And on the right is Lord Chaitanya, who is none other than Lord Krishna. Please read ‘Teachings of Lord Chaitanya’ to know more about Them.
Since They are very merciful, Their worship is also very simple and purifying. Best way of worshiping Them is to chant Hare Krishna Mahamantra in a prescribed manner and participate in the congregational chanting with all the devotees.