hard drive - How to add unused space to another partition in gparted?

In my hard drive windows takes up 100 gb, then backtrack takes up 100 gb. When I make backtrack's partition smaller i get 100 gb for windows, 50 gb for backtrack, and 50 gb of unused space (in that exact order). How do I reallocate that 50 gb of space to windows so that instead it is 150 gb for windows, then 50 gb for backtrack? I'm using gparted and i can't move the unused space or add it to windows' partition.