Doom engine legality

To be completely honest, I do not believe that anything will come of this; it would be more effort than I really think I could put into it. But I am still curious, and thought I would at least ask about it.

What would be involved with someone making a Doom mod and then selling it? (Legally.)

I mean, the Doom source code has been released, so for the sake of argument, let's say I make a few alterations to the source code in some way to make a "modified Doom engine" (But you know, really it's not very modified,) and then replace all the art/sound/map assets with something original. Would I still need to be paying royalties or getting permission to sell that product? (And if so, who would I need to talk to, and does anyone know how likely they are to let me sell said product?)

And second to that, how would these legal questions come into play if I were to try to start from a source port so that the game already runs on modern systems? Would I need to get permission/pay royalties to the people who did the work of porting the game over? Are there some source ports where the people who have worked on it have already given approval for folks to use it as they please, even for commercial products?
I wouldn't want to cut someone out of their dues, but I would want to cut down on the red tape I'd have to go through.

Any Doom source port released under the GPL license (i.e., almost all of them) can be taken and used commercially, provided of course that you don't use any copyrighted art assets or whatever. The only requirement is that you also freely release the source code for your game under the GPL in turn.

What Ling said.

Case in point; there are two games using 3DGE being developed and published independently for Sega Dreamcast that will be for sale; SLaVE and Hypertension - Harmony of Darkness. No plug intended. (since the first of those two is kinda my baby :P )

Neat. That's really cool to know.

Now if only making an entire game's worth of art and maps was actually as easy as it looks in my head.

http://forum.zdoom.org/viewtopic.php?f=4&t=52007&hilit=Legality

Linguica said:

Any Doom source port released under the GPL license (i.e., almost all of them)

Almost all of them??? o_O

^ http://doomwiki.org/wiki/Comparison_of_source_ports#Comparison_by_license.2C_platform_and_supported_games

Curious, nobody's mentioned how GLOOME is a thing.

Is a thing or was a thing?
Gloome hasn't seen any updates for 7 months so it's quite behind the current ZDoom mainline.

Graf Zahl said:

Is a thing or was a thing?
Gloome hasn't seen any updates for 7 months so it's quite behind the current ZDoom mainline.

While that's a fair point it's still a usable, commercially viable engine.

Oh! Yeah, that Gloome looks pretty slick for making something completely original!

Not updates in seven months? Eh, that's still considered alive in my book.

There's always Nash's GZDoom-GPL, which has the benefit of being up to date with its parent port.