Thanks for all your help guys,
"ppm install http://www.sisyphusion.tk/ppm/GD.ppd --force"
also caused an error for some reason, so I just erased what I had and replaced it with strawberry perl, and I got to do what I wanted to.
Well ... that was easy. Everything's working fine this morning - no need for me to take any action at all.
I have no idea what was going on. The error message's reference to a ".tgz" file instead of the more usual ".tar.gz" is a little suspicious. I'm guessing the web hosting company did something that led to the problem ... and then subsequently un-did it.
No good asking them about that - they're as secretive as a national security organisation, and as ruthless faik.