You can't re-create anything that is clearly our work.

You can make a completely different game using the code with our blessing,
but if it has imps, cyber demons, BFGs, etc, then you are treading on thin ice.

I hate to be the one to tell people this, but the reality of the business
is that we are obligated to defend our trademarks and copyrights, so we
can't just let things like that happen, even if they aren't perceived as a
"threat".

You could make and release a new DOOM wad that replaced everything with
similary looking original art, but it would still be bound by the
restrictions we place on community derivative work, so it wouldn't be
"free" in the sense that the GPL'd codebase is.

John Carmack