For the standalone, Rockstar Warehouse version:
Hit "Start", "Run", and type "cmd" without the quotes. Hit enter.
Type "cd c:\Program Files\Rockstar Games\Grand Theft Auto V" without the quotes in the cmd window that pops up. Hit enter.
Note: If you’ve installed the game to a different directory, enter that directory instead.
Type "GTAVLauncher.exe -verify" in this window (no quotes again) and hit enter.
For the Steam version:
Right click the game in the “Library” tab.
Click “Properties”.
Click the “Local Files” tab.
Click “Verify integrity of game cache…”
Your computer will check all your files, this can take a while. After that, it must be solved. (it will recover corrupted files)