If building or using a package depends on other GNU packages, these will be built automatically in the correct order. To see the dependencies of any package use the dep-list target.
$ make -C gnu/gnupg dep-list make: Entering directory `/home/gnu/gsrc/gnu/gnupg' libgpg-error libgcrypt libassuan libksba pth zlib readline make: Leaving directory `/home/gnu/gsrc/gnu/gnupg'
The dependencies are searched for in the gnu, gnustep and gnome subdirectories by default. Of course, packages might depend on software that does not belong to the GNU project. In those cases, GSRC will try to determine whether these external packages are installed on your system. If one is not present, you will have to install it separately, for example via your distribution’s software repositories.
Note that the dependencies can be more than one level deep,
$ make -C gnu/readline dep-list make: Entering directory `/home/gnu/gsrc/gnu/readline' ncurses make: Leaving directory `/home/gnu/gsrc/gnu/readline'
So, to install a complex package like gnupg use the same commands as for a simple package,
$ make -C gnu/gnupg $ make -C gnu/gnupg install
All of the dependencies (and the dependencies’ dependencies) will be built and installed first, as needed.