PlayerCompanion is a mod for Grand Theft Auto V that aims to extend the gameplay features of the game by adding some extra tools for developers and users alike:
The main target of the mod is increase the replay value of Grand Theft Auto V by using PlayerCompanion in combination with other mods to increase the things that the user can do in the game.
Just drag and drop all of the contents of the compressed file into your scripts directory.
You can open the built in inventory menu by pressing I on your keyboard or entering the word "inventory" in the cheat input field.
Check the wiki for the usage instructions of the other functions that are part of PlayerCompanion.