Beat the game on any difficulty and you can activate these cheats in the 'Extras' menu.

Duke 3D Freeze Ray = Freeze ray will fire bouncing ice projectiles.
Game Speed = Adjust game speed.
Grayscale Mode = Draw the game in grayscale.
Head Scale = Adjust head size of enemies and NPCs.
Infinite Ammo
Instagib = Enemies die in 1 shot and always gib.
Invincibility
Mirror Mode = Draw the game mirrored horizontally.

Note: This will disable achievements and it's only for single player.

https://steamproxy.net/sharedfiles/filedetails/?id=1892583013
https://steamproxy.net/sharedfiles/filedetails/?id=2074013065