How can I gain screen share/ssh access to a G5 connected by ethernet to a Mac Mini on my wireless Airport Extreme Network from my MacBook Pro?
This is happening because the two machines are not on the same subnet. 

The G5 is on the Mac Mini's subnet, while the Mac Mini is on your Wi-Fi's subnet. The IP address of the G5 has been handed out by the Internet Sharing DHCP service on your Mac Mini, while the Mac Mini's IP address has been handed out by the DHCP service on your router. 

Your best bet would be to connect your G5 via Ethernet to your router. 

If that is not possible because of location constraints, there is a piece of software for Mac called IPNetRouterX that may do the job. It allows Ethernet bridging, which is what you want. The only problem with this software is that it costs money. It seems like there might be a free trial of it, however.