You can tell if a module is pure Perl or not by
examining the dslip information.
One module which does this is
CPANPLUS (it also does
a whole lot more, including module installation without
root access, which I recommend over including the
module source in your
default shell includes the 'l' command
which will give information, including the language used.
CPAN.pm might also give dslip info, but I don't know it well
enough to say if it does or not.
GD is C and Perl and a C compiler is needed. If your
ISP allows you to use a compiler, you should still be
able to install it without being root.
For instructions on how to do this with CPANPLUS,
refer to the FAQ (available on the website).