GnuPG for Apple's Leopard 10.5.x
The fact that your system is on Leopard rather than Snow Leopard (10.6) will make things a bit trickier. A fair amount of software is only compatible with 10.6 and above.

That said, I'd suggest installing GnuPG using one of the UNIX package managers for Mac OS. I see MacPorts still has a legacy installer for Leopard - see this link. 

Unfortunately there doesn't seem to be a binary for 10.5, only 10.6, so you'll need to build it from source, which the MacPorts port command will manage for you.