The unidentified nurse made a stand after deciding the practice was a criminal act, said Cori Crider, a lawyer for British legal rights group Reprieve.
Ms Crider said: "This guy is basically a hero and he should be permitted to give care to detainees that is ethically appropriate."
It is the first time a nurse or doctor is known to have refused to tube-feed a prisoner, said US Army colonel Greg Julian of Southern Command, which oversees Guantanamo.
He said the nurse, a lieutenant, had been assigned other duties at the centre.
Ms Crider found out about the case during a phone conversation with Abu Wa'el Dhiab, a 42-year-old Syrian prisoner she represents who has never been charged.