the ppm doesn't seem to be able to find libgd on my machine

PPM doesn't need to find libgd on your machine. (The binaries already incorporate the libgd functionality, so you don't need libgd on your machine.)

1) What was the exact ppm command that you ran ?
2) What was the exact error you got ?
3) What does running perl -v produce ?

Cheers,
Rob