[wang_li]

In the README.txt distributed with the release it says:

> Licensed under the GNU General Public License 2.0.

Which is the only text that puts it under the GPL at all. There is no later version mentioned.

https://github.com/id-Software/DOOM/blob/master/README.TXT

Regardless, you can't change the license on code you don't own, even if the code is distributed under a flexible license.

[rpdillon]

Don't read the readme to figure out what the license says. Read the license: the readme only tells you what license. It doesn't tell you the terms of that license.

The actual license used explicitly allows later versions of the GPL to be specified in place of GPLv2.

https://github.com/id-Software/DOOM/blob/master/LICENSE.TXT

> Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.