I use GParted myself too, but only ever used it on single partitioned disks on VMWare.
One thing I can tell you though, when it says to press any key to interrupt the disk check - leave it the hell alone and let it do its check.
I also echo the advice to back it up first.
Some other things you could do:
1) Move the page file to the other partition - It'll need a reboot to do, but is relatively painless.
2) Run Treesize - it might show up some deprecated profiles, huge temp folders or other hidden surprises you can easily remove.
3) Delete the pesky NTUninstall folders littering up C:\Windows - You won't be able to uninstall the corresponding windows updates after this so be really sure you aren't going to want to rollback - Maybe leave the last 3 months of folders if you update regularly.
Do this at your own risk - it's never caused me any problems but YMMV.