For some historic reason Gpg4win has an executable source installer containing all the dependencies and a source tarball containing none of the dependencies.
This probably comes from a time where a 300mb download was "huge" and as such it was decided to make the source tarball available independent of the dependencies.

I think this should be changed because you can neither use the source installer to rebuild Gpg4win nor the source tarball without further interaction / copying stuff around from Windows to Linux.

GnuPG-w32 includes all dependencies in it's source package so you can patch / rebuild GnuPG-w32 from the tarball without dependencies to external downloads directly without any copying.

Gpg4win should do that, too and drop the source installer.