I can understand the need for keeping secrets in any government. Revealing everything they know could compromise some intelligence assets, both here and abroad. An enemy who knows what we know could figure out how we came to know it. I don't have a problem with keeping those kinds of secrets. Or about strategic assets, such as the stealth program, which was extremely secret at one point, though apparently less so now.
But it's my understanding (and I have no first-hand knowledge of this) that the vast majority of information which has been classified "Secret" by any government, is done so more to protect the reputations and/or dignity of government officials, not to protect the security of the country. It's my opinion that keeping information regarding the activities at Guantanamo Bay secret is, for the most part, one of those kinds of secrets.