Remember to rename the PlayGTAV - like in the video - in the properties so it doesn't look like a pirated copy, as it will say "Non-Steam Game" underneath to other Steam users
If you are not the admin of the computer, you will need to enter the admin's password each time you start Steam
This will allow you to play GTA V through Steam, and use the Steam overlay, Steam Music and Steam Controller ingame.
This is useful if you have friends that have bought the steam version of the game, as they will be able to see when you are playing.
Feel free to add me on Steam, since my account name is on screen lol.