[Whistler]

have to going OT for yet another time:

"strip away all that you don't need and rename stuff here and there so that nobody can prove that you've actually used it - you won't be linking against the metamod source code, but just against the HL SDK"
...then is it legal if I use a exe resource editor to change all the texts and graphics in a PROPRIETARY software and release it for free ?

"And as the GNU people say themselves, a plugin of a GPL program doesn't need to be GPL unless the author of the main module explicitly wants it so."
Yes, it doesn't need to be GPLed *at all*. You can use BSD License for plugins if you want to. That is to say, you just need to grant all the permissions in the GNU GPL.
And if plugins is only called and does NOT make dynamic link to the main module you can even make it non-free.
Also Will Day DOES "explicitly wanted" it, just take a look at the Metamod faq file.

"...and certainly not to take away from the coders the freedom to disclose theirs or not."
that is to say, "and certainly not to take away from the coders the freedom to take away users' freedom to share and change the programs or not".

"But Whistler is a die-hard GPLer, one of those who could turn a license designed for freedom in one of the most restrictive licenses ever."
GPL IS "the most restrictive license" for non-free software developers; it's only good for users and free software developers who don't want others to use their code in non-free software.
Remember it is NOT designed for non-free software developers' freedom at all. So if you don't think non-free software is bad, just don't use GPL. No one has forced you to use GPL for all of your code; the point of GPL is to make sure all copies of the ORIGINAL software is always free software.