From the text of the letter:
We write regarding the forcefeeding and restraint of Guantanamo Bay detainees currently on hunger strike.1,2 The World Medical Association specifically prohibits forcefeeding in the Declarations of Tokyo and Malta, to which the American Medical Association is a signatory.
Fundamental to doctors' responsibilities in attending a hunger striker is the recognition that prisoners have a right to refuse treatment. The UK government has respected this right even under very difficult circumstances and allowed Irish hunger strikers to die. Physicians do not have to agree with the prisoner, but they must respect their informed decision. Those breaching such guidelines should be held to account by their professional bodies. John Edmondson (former commander of the hospital at Guantanamo) instigated this practice, and we have seen no evidence that procedures have changed under the current physician in charge, Ronald Sollock.3
Edmondson, in a signed affidavit, stated that "the involuntary feeding was authorized through a lawful order of a higher military authority."4 This defence, which has previously been described as the Nuremberg defence,5 is not defensible in law. In a reply to an earlier draft of this letter, Edmondson said that he was not forcefeeding but "providing nutritional supplementation on a voluntary basis to detainees who wish to protest their confinement by not taking oral nourishment".
The affidavit is filed in Al Joudi et al vs George Bush in the US District Court for the District of Columbia. Case1: 05-cv-00301-GK. Document 48, Exhibit A. Filed Oct 19, 2005.
There's also some personnel screening going on at Guantanamo. The letter says:
Recently, it was confirmed that health-care staff are screened to ensure that they agree with the policy of forcefeeding before working in Guantanamo Bay.
The cite for the claim is listed as Okie S., Glimpses of Guantanamo: medical ethics and the war on terror. N Engl J Med 2005; 353: 2529-2534. In the article, which is a first person acount of doctors touring Guantanamo, the author reports a conversation with Captain John S. Edmondson, an emergency physician and the commander of the medical group that delivers the prisoners' care:
The military's policy of tube feeding prisoners on hunger strike is controversial, and military health care providers are "screened" before deployment to Guantanamo "to ensure that they do not have ethical objections to assisted feeding," Edmondson told me.