Now, I am not so sure that I agree with this but under Congressional Acts related to the formation of Department of Homeland Security, Trump does not even need to have a Court Case (at least not outside DHS) if there is proof of Garcias actions in MS-13.
This is related to rulings from the War of Terror era where the US Government could apprehend and even deport members of Al Quadi and other terrorist groups. The Trump administrations declaration of MS-13 as a terrorist group was strategic.
It is not well known but DHS has an internal court mechanism that was uniquely created for them to rule on terrorism threats.
Now, I think there is a solid argument that DHS and all the acts around it are unconstitutional and I can join you in that argument but that is the current state of law we are in.