Transferring photos from G5 to iMac

Unfortunately, Photos no longer opens iPhoto Libraries since, I think 10.14.6 Mojave. and macOS 10.15 Catalina, is newer than that, so there's a no-go there.

Photos can Import pictures from an iPhoto Library, but you lose all organization events. If that's not a problem then, when you copy the iPhoto Library to a drive that can be read by Photos, you can just use Photos' menu File>Import to navigate to the iPhoto Library and import its pictures into the Photos Library.

If the iPhoto Events organization is important to you, then the best bet is to use the trusted 3rd party app PowerPhotos ($40) which can import or merge Libraries, including iPhoto, while maintaining events and albums and folders. You can look here for more information:

PowerPhotos converting iPhoto Library

Copy the library from the old machine to an external drive. Then move it to the new machine. Open it with Photos.

IMO the best method is to use the paid version of PowerPhotos to merge an iPhoto library into a Photos library. This user tip compares Photos vs PowerPhotos regarding merging. It also describes what PowerPhotos can do in importing: Merging/Importing iPhoto/Photos library into Another Photos Library: PowerPhotos vs Photos. It also is the premier utility for Photos libraries on the market.