If you have an internet connection:
To install GD or Net::SNMP, use
ppm install GD
ppm install Net::SNMP
after first setting the proxy variables, if you use a proxy to access the Internet (you will need an Internet connection for this to work). See the PPM documentationon the ActivePerl website for details.
If you are unable to install GD by the above method but you still have an internet conenction, try this instead (perl 5.8):
ppm install http://theoryx5.uwinnipeg.ca/ppms/GD.ppd
or, as a last resort, this (perl 5.6, maybe perl 5.8):
ppm install http://www.steveshipway.org/software/3rdparty/GD.ppd
If you don't have internet access from the server:
Finally, you can download the files http://www.steveshipway.org/software/3rdparty/GD.ppd
then modify the ppd file appropriately for your system, or even install the ppm file (inside the .tar.gz) directly.
Thanks to Gary Phillips for the necessary information!