hmm I don't think you are allowed to resize an active partition .I'm assuming you have a dual boot , correct?
You windows os will have the settings greyed out and your Linux distro won't allow you to use gparted because it's an active partition .
Steps
1) Visit Gparted website . Remember you want the live image.
2) get a spare flash drive and create a bootable flash .You can use a disc if you want.
3) from your Windows machine use either Rufus or USBinstaller .(If the latest version doesn't allow you to create a bootable flash - use the unsupported option and it will work .
4) power off machine .Boot from flash .Wait for Gparted to power up .
....
Active drives can not be resized or modified while it is active .So we power the machine down 1st .With your machine off you will be able to use Gparted to create /modify existing drives .
Hope this helps .All the best .