HOW can I make a if condition

Overriding vanilla GUI is tricky, but not impossible. You'll need to make a procedure that triggers when the player right clicks on an Ender chest. (Use the global trigger 'player right clicked on block,' and then use an if statement to check if the block is an enderchest.) 

Have the procedure close any GUI the player currently has open, then immedieately open your custom GUI. This works most of the time, though there is a bit of a delay.