If your PC is weak, look for "Potato Mods" on GTA5-Mods that reduce graphics settings rather than shrinking the installer file itself.
The game frequently goes on sale for under $15, providing a safe, verified 100GB installer. gta 5 highly compressed 500mb for pc link
To understand why a 500MB version of GTA 5 is almost certainly fake or dangerous, you have to look at the game's actual size. If your PC is weak, look for "Potato