Create a Shortcut file of your GTAVLauncher and save it somewhere other than your game directory. Right click and go to Properties. in the Target box, it will show a directory to the .exe file, ie: "C:/ProgramFiles/RockStarGames/GTAVLauncher.exe" - after the end quotation marks, put in a space, then "-verify" (with quotation marks.)
So in the Target box it should say something like: "C:/ProgramFiles/RockStarGames/GTAVLauncher.exe" "-verify" Hit Apply/Ok and run the launcher. Should verify your game files and download any corrupted ones, then the game should boot.
Let me know if this worked.