Well then... I don't think that Obama would close Gitmo releasing the detainees to go as they please, and I don't think that the vast majority of people who are in favour of closing Gitmo would think that that would be a likely consequence. I think it's a rather stupid strawman by fear-mongering rightwing nutjobs with a paranoid gun obsession who, even as Obama has won the election and is about to be inaugurated as President of the United States, can't help themselves over their Pavlovian reactions to all things Obama and have to peddle the "if Guantanamo is to be closed, all the detainees were to go free as they please" idiocy.
I think the vast majority of people who are in favour of closing Gitmo are mostly in favour of ending a system that was purposefully installed to operate outside of the law, in violation of international treaties and in violation of the United States Constitution, and they would be perfectly fine with trying and sentencing terrorists, provided such a case can be made in a court of law, to sentences similar to the one 9/11 plotter Moussaoui has received.
I also think people would be in agreement with putting bin Laden on trial in the United States, provided he'd get caught. I don't think any speculations on whether or not trying to catch bin Laden would be any more successful under Obama than they were under Bush have any merit.