I doubt that Glenn Cramer is subject to an FBI "gag order" but after a little googling it seems it *may* be possible after all without a courts approval.
"Three years ago, I received a national security letter (NSL) in my capacity as the president of a small Internet access and consulting business. The letter ordered me to provide sensitive information about one of my clients. There was no indication that a judge had reviewed or approved the letter, and it turned out that none had. The letter came with a gag provision that prohibited me from telling anyone, including my client, that the FBI was seeking this information."
http://www.washingtonpost.com/wp-dyn/content/article/2007/03/22/AR2007032201882.html
A National Security Letter (NSL) is a form of administrative subpoena used by the United States Federal Bureau of Investigation and reportedly by other U.S. Government Agencies including the Central Intelligence Agency and the Department of Defense. It is a demand letter issued to a particular entity or organization to turn over various record and data pertaining to individuals.
They require no probable cause or judicial oversight. They also contain a gag order, preventing the recipient of the letter from disclosing that the letter was ever issued.
http://en.wikipedia.org/wiki/National_Security_Letter
But, these National Security Letters are defined as being "issued to a particular entity or organization to turn over various record and data..." so it seems unlikely that they could simply be used to silence someone from talking about an event.
The problem is you can't say you have received one. So, even saying you're "gagged by the FBI" would probably be a violation.
Nice catch-22.
Edit: I just finished reading the rest of the Wiki article on NSL's. I had never heard of these things before. Very interesting.
However, I still call BS on the OP. IF they DO gag you, you can't even say so!