Senator Ben Sasse (R-NE) issued a scathing a press release in reaction to the President's plan to confiscate guns prior to due process considerations. If Trump were to actually do this (it's doubtful), it would be rightly seen as an act of a tyrant.
The U.S. constitution explicitly guarantees due process to Americans in both the fifth and fourteenth amendments.
Legally, the President could not unilaterally override the fifth and fourteenth amendments without a constitutional amendment. Trump, being Trump, probably did not know this.