The spec is OpenPGP and both PGP and GPG implement it: https://tools.ietf.org/html/rfc4880
I'm surprised those software actually use GPG by parsing its output instead of as a library.
With that, for GPGME to be LPGL there can't be an actual coupling of code between the two or else GPGME would have to be GPL licensed itself. (Though you could argue the distinction here a bit murky)
Since quality CLI implementations must be provided and exist anyways, why not provide the programmatic interface (libgpgme) as a thin veneer over it, as if the commands are being run from the shell?
The major advantage of GPGME is that it's written by the same folks as GnuPG itself, and hopefully they know how to use the binary correctly.
If we are being pedantic, that detail is important.