[tgma]

Importantly, GPL does not include such line. That line is not part of the GPL. Critical software such as Linux famously do not have that and are not GPLv3 for example.

The Author can choose to license the Copyrighted Work under various licenses. The common boilerplate that many Authors of GPL software choose to use is a boilerplate that says they are licensing their Work under GPL version X or any later versions supplied by FSF. That boilerplate is Author's declaration of license contract and not the license itself.

Basically the Author is directly saying (not through the GPL text) that "I trust FSF to come up with future licenses and call whatever they want GPL version Y [Y>X] and I'll be willing to license my work under that license sight-unseen also."