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 .
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 .
The OP (that's you, Chris, the Original Poster) already has a Live GParted at his fingertips, namely on his USB Live install stick for his Linux Mint 19.1 'Tessa' Cinnamon.
So there is no need to download it from GParted's site, although that being said, it is always handy to have the standalone GParted (and labelled) , where Distro sticks can be overwritten and reused once an install is successful.
Chris also has two (2) other recent and current Threads HERE and HERE ... that explain his circumstances. Be warned the wifi one is 3 pages long, but now hopefully resolved (eh, Chris?)