How do I target a specific instance? For example, if GlobalVariable(Keys) == 1 then destroy nearest/first instance of object Door. Is this even possible?
If you know wich instance you want to target, it’s easy:
-Add an object variable (e.g. “ID”) to the desired instance and set a non-zero value (e.g. 1).
-Then add the condition
Variable "ID" of Object is = 1
So Game Develop will know you want to target that instance 
(The ID variable should be different than zero because if you don’t set the ID variable in others instances, GD will set ID = 0 for all of them)
If you want to know the nearest object, check this 
[url]Find object nearest to a point]
or this:
[url]Testing for nearest object - #10 by Lizard-13]