According to the transcript, only the frame of a Glock-style pistol is considered the "gun" under US gun control laws.
The transcript mentions that some filing was done to a "slightly crappy commercial part," not the 3D-printed part itself, to improve the gun's function with the suppressor. Additionally, modifications were made to the barrel and the slide to improve the gun's function with the suppressor attached. The exact nature of these modifications is not specified.
This WIRED video details the process of 3D-printing a ghost gun, the same model allegedly used in a high-profile assassination. The video explores the legality of 3D-printed firearms, the evolving technology, and the ongoing debate surrounding gun control in the United States.