Comment by seabass-labrax

> Hah. Well it is now...just copy & paste the GPL's text and append that one condition.

Section 7 of the GPL (version 3) explicitly gives the user the right to remove that additional condition. So if you want it to be effective, you will at the least need to remove section 7 from your copied GPL. Then, you'll need to remove the preamble and instructions to avoid the Free Software Foundation's ire (they're being generous in allowing you to modify the license test at all).

Ultimately, it goes against the spirit of the GPL so much (and against the point of FOSS in general) that it would be entirely unjustifiable to use the GPL as the foundation for such a proprietary, source-available licence.