Cool, cool!
So where do you store your items? I see “Gained Items”, do they stay there?
Basically, on interaction with something requiring a key, you would have the thing being interacted with (chest, door, etc) ask the character if they have a key first. Here that’d be something like
Other Actor CharacterReference →
GET KeyCount → >=1? (bool check) with a branch
False path printstring “Need More Keys”, True path do your code to get the item, drag off the character reference again and SET Key Count -1 just like you do on the Gain Item event, just subtract instead of add! 
Hope that helps! Let us know how it goes, or if you hit a snag!