Zhisheng is a prominent Christian human rights lawyer best known for his work defending persecuted social groups but was sentenced to three years in prison charged with "inciting to subvert the state power" in December 2006.
It's claimed he was severely tortured in detention before being released on probation.
In December 2011 Chinese authorities claimed Gao had violated the terms of his probation and had been returned to prison for three years.
According to a statement by the US-based Freedom Now Mr Zhisheng's teeth are in a poor condition, likely due to malnutrition and there are concerns about his mental health.
The organisation said: "Gao was held in a small cell, with minimal light, 24-7-365. Guards were strictly instructed not to speak with him.
"He was not allowed any reading materials, television, or access to anyone or anything.
"He was fed a single slice of bread and piece of cabbage, once a day; as a result, he has lost roughly 22.5 kg (50 pounds) and now weighs about 59 kg (130 pounds).
"He has lost many teeth from malnutrition. It is believed he was also repeatedly physically tortured.
"Unfortunately, it is hard to get much more than basic information from him. Gao has been utterly destroyed. He can barely talk — and only in very short sentences — most of the time he mutters and is unintelligible.
"It is believed he is now suffering from a broad range of physical and mental health problems; he has not been allowed to see a doctor since his release."
The lawyer has been unable to see a doctor and his wife is calling for him to be allowed to travel to America for treatment.
Christian Solidarity Worldwide's Chief Executive Mervyn Thomas said: "These reports of Gao Zhisheng's ill-treatment in prison, while not unexpected, are nevertheless extremely alarming.
"While we welcome Gao's release from prison, we condemn in the strongest terms the physical and psychological ill-treatment he appears to have suffered.
"We urge the Chinese government to allow him access to medical care and contact with his family and friends.
"We further call on the authorities to guarantee his rights to freedom of movement, association and expression, as provided for in the International Covenant on Civil and Political Rights which China signed in 1998.
"Finally, we call on the government to ensure that any reports of torture in detention concerning Gao Zhisheng and all other prisoners are investigated thoroughly and impartially, and that those found to be responsible are held to account."