There are quite a few versions of the G5 iMac's, I don't remember if they all have a universal power supply.
The best way to find out is to review this listing [https://everymac.com/systems/apple/imac/index-imac.html|EveryMac - iMac's] locate your exact model, then open the page fully so you can then jump to the Ports tab section and lastly double click on the very last entry ***Power*** you should then see this: ***''Details:100V-240V AC, 180W maximum''*** If its dual Voltage.