Sounds like you didn’t format the drive to the correct low level format. The G series use an older format type than what the Intel systems use (GUID/HFS+).
You also need a SSD which supports SATA I (1.5 Gb/s) unless the spec sheet calls it out the drive won’t work! Luckily the Samsung 860 EVO is one of the few that should work.
Apple never validated SSD’s with the older OX-X’s, El Capitan is the first release Apple offered TRIM support for 3rd party drives which doesn’t help you.
You’ll need Cindori’s [https://cindori.org/trimenabler/|Trim Enabler] for the OS-X Leopard. I’m not sure if they offer a compatible version for the G series version OS’s.