You have to select the correct device using the tab top right (currently set to your USB device which is mounted as dev/SDA in the picture) on Gparted. It can only handle one device at a time (but will show all partitions on that device). It is defaulting to the device that Linux booted from, the USB stick.
The program called "Disks", an app, is a bit less fiddly, in that you can see all the devices available without clicking through a list of them. Although again it will only select one device at a time and limits adjustment of partitions to the selected device.
EDIT: Changed "drive" to "device" for clarity. Click the device selector top right. you should see your main windows hard drive available as well as the USB stick.