Download this here http://download.cnet.com/MD5-SHA-Checksum-Utility/3000-2092_4-10911445.html
Once it's booted up, Hit browse. Find each of the files one by one (in your Gta 5 folder) , copy paste the long codes in the box next to verify. If the program tells you they don't match, delete that file.
Mine was x64s.rpf as the corrupted file, yours may be different though
I'm leaving this here for reference:
If that didn't solve your issue, be sure that there is a file named "appmanifest_271590.acf" in the Steam\steamapps folder of the drive you want the game to be recognized on. If not, here is a download link to my appmanifest_271590.acf file (as of today, 5/13/2015). Whether or not using mine will work, I do not know. The reason I say that is because there is "LastOwner" in the file followed by a numeric string. If you open the file with a text editor such as Notepad++ or Sublime Text you will see this.
By default everything was set to either high or very high. And I tried turning everything low but it did not help. This is my card if it helps