President Obama as chief executive of the country is attempting to finalize one of his campaign promises, made in 2008 to close GITMO. He first tried in his first administration and fear of bringing these prisoners to the US stopped him and the Congress attached a rider to a bill forbidding the spending of any funds for transferring to the US. So yes there is a law or at least a financial prohibition.
As to the rest, you are badly misinformed or have ODS. Here are some facts:
349 convicted terrorists are already being held in US Max security prisons. These are people who have been tried and convicted. They are were afforded trials and a chance to tell their side of the story, to confront their accusers and plead their case. That is what America stands for, duh. None of these terrorists has escaped. No American is at risk because they are jailed here rather than in Cuba.

Gitmo originally held about 800 prisoners. Some were caught up in sweeps and not found to have violated our regulations and were sent home. Some were judged to be terrorists but were sent back as they we were done with them. Of course none of these men had open trials in Gitmo.

George W Bush administration released 532 detainees. Read that again. Under Bush 532 were released. Of those 35% are known or suspected of rejoining the fight. Under Obama 121 have been released and only 10% are known or suspected of rejoining the fight. Apparently the Obama administration is doing a better job of evaluating the risk than the previous President's team did.
There are now 91 men left at Gitmo. 91. Of these 35 are in administrative hold. That means we are done with them and ready to send them "home" but there is no place to send them at this time. When detainees are released there has to be a country that says... Send them here... These 35 are awaiting an offer.
So that leaves 56 detainees we wish to continue to hold. Is this country so weak and our max security prisons so poorly run that they could not be trusted to hold 56 people? We are already holding 349 of them quite successfully. I know fear sells well in politics. It sells well in the Middle East also where fear of the US brings more recruits to ISIS and others. And one of the proofs they present is our own country's willingness to hold without trial, without charges, without time limits, without due process Muslims in military detention camps away from the media and family. If they are guilty try them, convict them and lock them up.
What is there about the GOP that turns off their brains and makes their mouth yell "NO" to whatever Obama suggests? What do you fear would happen if we closed GITMO other than save Millions of dollars a year.
You think I made these numbers up? Sure you do.
https://en.wikipedia.org/wiki/List_o..._Bay_detainees