Since this is a PS2 ISO, you cannot run it natively on Windows. You must use a PlayStation 2 emulator.
If you want to save space without risking malware or broken games, consider: God Of War 2 Ps2 Iso Highly Compressed For Pc
None of these get you to a "900 MB final game" because that is mathematically impossible without removing gameplay. Since this is a PS2 ISO, you cannot
Once downloaded (e.g., as a .7z, .rar, or .zip file), use 7-Zip or WinRAR to extract it. The output will be a .iso or .bin/.cue file around 3–4 GB in size. None of these get you to a "900