If you like what I do, please consider supporting via Patreon! http://www.patreon.com/shaunjs
Download the project files! http://www.gamemakertutorials.com/tutorials/inventorytutorial.gmx.zip
This is a big one! A lot of this tutorial was based on a lecture I gave on similar topics at Teesside University. The idea is to show you the fundamentals behind setting up almost any kind of inventory system using Arrays to store items, how to go about drawing it to the screen and how to use scripts to add, remove and check for items in the inventory.
I recently released a game on Steam that I built with GM:Studio! http://shaunspalding.co.uk/AnotherPerspective/