Is GZDoom on the list of "Unity Ports"? Also, in GZDoom you are able to jump. Is this allowed? And one last question - Do you have to complete the game with the "it wasn't supposed to end this way" screen and still be a valid run, or do you have to get the completed level screen?

Yeah pretty much what Ducky said. Nobody considers GZDoom a valid speedrunning port since it has so many extra features that makes it it's own thing and isn't associated with speedrunning. The Unity port is the port you can get on Steam, Bethesda Launcher, Xbox One/Series, PS4/5, Switch, Android, Apple, and the Xbox Marketplace for PC.

To answer what Ducky was unsure of (end game stuff) You don't need to show the credits, but showing the results screen at the end of the run is required.

What I’m talking about for the ending (it wasn’t supposed to end this way) is if you die after you step on the teleport pad in the last level, it’ll show the ending text, not the stats screen. Is THIS way of ending the run valid?

@HarrisonDotNET That doesn't happen on the Unity port. If it's happening to you, you are playing the wrong port.