The Roberts Court is siding with the US Constitution, and so is the Trump administration.
The weirdest aspect to this story is how Trump critics are cheering for authoritarianism, and against the rule of law, without realizing it.
The US Constitution is very clear on the separations of powers. The Judicial Branch, including the Supreme Court, has no say over foreign affairs. It is not in the court's allotted powers. Per the US Constitution, they have no voice on such matters. Trump critics want unelected judges to exercise power over Trump's handling of foreign affairs, and that is what makes Trump critics champions of authoritarianism. They want judges to be all powerful rulers who are immune from elections and can dictate anything.
The El Salvador citizen currently sitting in an El Salvador prison broke US law when he crossed the border illegally. The Executive Branch has the constitutional power to expel the criminal foreign national. Garcia's terrorist gang involvement is irrelevant, and a catch-22. The reason there was a prohibition against deporting Garcia to El Salvador was because there was a rival gang who tries to kill members of Garcia's gang. If Garcia isn't a member of the gang then he can be deported to El Salvador, and if he is a member of the gang then he's a designated terrorist who should be deported. Either way, the correct and lawful course of action is to deport the criminal foreign national. None of that is authoritarian.
It would be dangerously authoritarian for the judicial branch to order the executive branch to go overseas to El Salvador, take possession of the citizen of El Salvador, and bring him to the United States. Such an order would violate limits on jurisdiction and separation of powers which is why Robert's court didn't order that. Robert's order isn't a violation of the US Constitution, and Trump's administration doing nothing more than offering a free airline ticket back to the US if Garcia shows up at the US embassy in El Salvador to claim it also isn't a violation of the US Constitution or Robert's court order.
STOP CHEERLEADING FOR AUTHORITARIANISM!!