The GPL does contain an advertising clause, of sorts: clause 2c. However, this doesn't require that you name your program GNU/anything, nor does the FSF claim that it does.
RMS's rather quixotic one-man campaign to rename Linux-the-OS (as opposed to the GPLed Linux-the-kernel, which he has never suggested renaming) is not required by the license. He just thinks it's a way of giving the GNU Project credit.
Other people think this is silly: and perhaps it is (I take no stand on this myself: people can name things whatever the hell they like as far as I'm concerned). But legal concerns are not involved.
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