If a website claims to offer a GTA 5 setup file that is only a few megabytes or gigabytes, it is physically impossible for the actual game data to be inside. The Common Risks Involved:

The direct application from the developers of the game.

Right-click the .exe file, go to Properties , and check the Digital Signatures tab. If it doesn't say "Rockstar Games" or a trusted distributor, do not run it.

Before investing time in downloading, verify that your PC can run the game. Rockstar Games has published official requirements for both the Legacy and Enhanced editions.