Go to a trusted repack site (e.g., FitGirl Repacks). Search for "Grand Theft Auto V."
If you want to play GTA 5 on a low-end PC, you should ignore the PS3 emulator route entirely. Instead, look for the Official PC Port that has been "repacked" by trusted scene groups (like FitGirl, Dodi, or ElAmigos).
Download RPCS3 from the official website. It is open-source and free. Avoid downloading emulators from random YouTube links, as they often bundle malware.
Here is the hard truth that many download sites won't tell you: GTA 5 cannot be compressed to 30MB or 500MB and still be playable.
While it is possible to compress game textures and audio to reduce size (perhaps down to 20GB-30GB), files claiming to be under 1GB are almost certainly fake. They are usually clickbait designed to generate ad revenue or, worse, contain viruses and malware.