Before answering, I've got to mention that cracked/pirated games are illegal ya Marwan
Now that you have legitimate serial, there are 2 possibilities:
1- The serial you bought is for the
steam version of the game.
=====> In this case, you're gonna have to redownload the whole game through steam.
2- The serial you bought is non-steam, i.e. the serial is taken from a disc (hard-copy of the game).
=====> In this case, you will probably be able to use the game files you already have. Just download the latest update for gta v, and install it (without applying the crack), then run it. It will ask you to enter a serial number. Enter yours, then enjoy.
Note: Most serials on amazon are for the non-steam version.