[crististm]

This is not such a gray area as you think. Search for the readline license and what that meant to clisp.

Hint: derivative work of a GPL code is GPL (although that was slightly different for clisp).

You are free not have GPL stamp on your code as long as you keep it to yourself. If you distribute it to a third party you are automatically bound by GPL.

[Dylan16807]

There is no gray area. Derivative works are GPL or illegal. But nothing physically forces people from breaking the law. You cannot GPL a piece of code against the author's will.

[icebraining]

Nitpick: against the copyright holder's will. They're often not the same as the author (e.g., work for hire).