(credits to @rulz from the GB Studio discord for this idea, he rules!)

* The template, step-by-step guide and rom were updated to allow for the usage of variables to hide or show items.   

HOW TO HAVE YOUR ITEM BE SHOWN ACCORDING TO A VARIABLE:

1.  On the Inventory's start script, place a 'Hide: Actor (item x)'
2. Then create a new Variable (let's name it 'Received Item x');
3. Then when this Variable is set to True (by receiving it on a quest or something) we also have it Show on the Inventory screen;
4. To do this, set a "If Variable is 'True': 'Received Item x'" on the Inventory screen, and inside it have it Show Actor: (item x). Otherwise the item won't know if it is supposed to be shown or not.

That's it!

Keep creative, folks!