Use the upper-right dropdown menu in GParted to select the correct physical drive. If you see a little padlock icon that means the filesystem is mounted. You can try unmounting it in GParted with the right-click menu, but sometimes this doesn't work. Your particular desktop environment or window manager may have its own graphical tools for mounting/unmounting filesystems. The safe and sure way is the command-line commando way:
# umount /dev/sdc1
GParted tells you the correct device name to use. Then click GParted -> Refresh Devices, and then you can partition and format.