You would think this administration would jump at the opportunity of showing the media any proof that Abrego Garcia was a member of any gang, no matter how circumstantial or weak the proof is. But I've yet to see any of it.
Congress does have the power (article I, section 8), however it delegated this power to the president for "extraordinary" situations with certain laws.
Also, other users showed that his deportation was suspended as he had been threatened by the gang members you allege he's a member of.