How to use merge windows unallocated space into Ubuntu using GParted?

Notice on the lef hand side of the partitions the little "key" icon. That means that the partition is locked and that's why you can't resize it. The reason it's locked is because it's mounted; it's mounted when you boot.

Here's the easiest way to do what you want: boot from a live Ubuntu CD which won't automatically mount the hard drive partitions. When you open parted, you'll notice that the key icons are gone. Good! Now resize the partition.