G5 will not run Lion, G5 were ended in 2006 PowerPC IBM inside.
Mac Pro came out in 2006 and will run Lion and also Tiger.
Trying to install Tiger has to be the 'right' version for the machine, retail was G5/PowerPC only while Mac Pro (Intel) such as MacPro1,1 needs the OEM DVD of 10.4.7 and up. The later G5 Quad and PCIe graphics required 10.4.3 or .4 I think
OS minimum build:
Community for PowerMac (G4, G5):
Also if the machine was upgraded with graphic card it may require a later OS than what it shipped with.