The organisation has demanded an explanation from the Ministry of Home Affairs.
Gray-Block was travelling from Sydney and was barred from entering into India on Saturday.
"No formal reason was given by the immigration officials for the decision and he was not officially deported," it added.
He was travelling to India on an Australian passport to take part in a series of meetings with staff and to learn more about Greenpeace India's current campaigns, it added.
Seeking a "full explanation" from the home ministry, Divya Raghunandan, Greenpeace India programme director, said: "Our colleague has a valid business visa, and yet he was prevented from entering India with no reason given."
"We support the free movement of people across the world, which is crucial to the work of business as well as charities. Greenpeace International is a global organisation that helps to find solutions to environmental problems. There is absolutely no reason why one of its staff members should be treated in such an arbitrary way," she said.
"We are forced to wonder if all international staff of Greenpeace will now be prevented from entering the country? If Home Minister Rajnath Singh has a stand on this, then we would like him to state it clearly," said Raghunandan.
The NGO added that Gray-Block was denied entry, his passport was seized and he was put on a flight to Kuala Lumpur.
"His passport was returned to him after he landed in Kuala Lumpur. He is now back in Australia. This is not the first time Greenpeace staff from other countries have been denied entry into India," the statement said.
It added that Greenpeace was a global organisation functioning in more than 50 countries.