Critics say that the method is antiquated and could encourage gun crime.
"The firing squad is archaic, it's violent, and it simply expands on the violence that we already experience from guns as a society," said Bishop John C. Wester, of Utah's Catholic Diocese.
Gardner's lawyer said that his client was coming to terms with the sentence. "I don't think it was a shock or a surprise, and he's coming to grips with that," Andrew Parnes told the Associated Press news agency.
He said he would launch last-ditch appeals, but admitted that Gardner's death looked "closer than ever before".
Utah is the only US state to use the firing squad, where the prisoner is shot by five marksmen, as a method of execution, although it is theoretically possible for Oklahoma to employ it if other methods are unavailable.
Until 2004, death row inmates in Utah were able to choose the method by which they were killed, but the state changed the law to make lethal injection the default method after facing criticism for a previous firing squad execution.
Because Gardner was sentenced to death before the change in the rules, he was still allowed to make the choice.
Gardner was convicted for killing a lawyer during an audacious escape attempt from a Utah courtroom in 1985.