Grab a copy of a umd dumper/emulator and dump ur copy of gta to ur psp mem stick (Might need a 256 or higher mem stick size). After that try and load the dumped iso using the umd emulator. Post any results here. If you have socom try that out too.
You can try Fastloader since it has an ez to use dumper/emulator (Dont let the "Erasing Flash" mesage fool you on the dumper part!).