The current documentation claims that the "ustar" format is used. However T5754 and other commits show that we gpgtar already creates a format that uses extensions of the ustar format like the pax headers. gpgtar can do larger files long unicode filenames and more.
So it is closer to posix in https://www.gnu.org/software/tar/manual/html_node/Formats.html#Formats
But the documentation doc/tools.texi still has
texinfo @command{gpgtar} encrypts or signs files into an archive. It is an gpg-ized tar using the same format as used by PGP's PGP Zip. @item --create @opindex create Put given files and directories into a vanilla ``ustar'' archive. @item --extract @opindex extract Extract all files from a vanilla ``ustar'' archive. @item --skip-crypto @opindex skip-crypto Skip all crypto operations and create or extract vanilla ``ustar'' archives.