This unnamed submachine gun is due to appear in Grand Theft Auto VI and appears to be modeled primarily on the real-world Heckler & Koch MP5/40. Both protagonists, Lucia and Jason, are shown carrying it during a hostage situation in the game's second trailer at around the 1:15 mark, with Jason's version notably fitted with a red-dot sight. No manufacturer name, capacity, or other specifications have been revealed for it.
Weapon data via GTA Wiki (Fandom), CC BY-SA 3.0.