Please post a screenshot from gparted !
I was able to implement what you wish to do - resizing my root partition with gparted and it went well. However, you need to back up in a very precisely correct manner and evaluate whether you really need to take from a /home partition which is already too small.
"What should I do" is of course the intuitive question but it is a bit broad so tailoring an answer is not quite as straightforward. Your goals and objectives are not entirely clear.
If the problem is that you are cramped in your root directory, you might simply delete unecessary software, as a start, and that will tide you over until you can add another disk drive or properly research your options, backup everything, and implement a plan.
Without more detailed picture of the status of the computer and what your purposes are, we must accept your apparent presumption that it is correct to cannablize your /home partition to beef up your root.
That seems to be robbing Peter to pay Paul and not necessarily a solution. It is theoretically possible to use portions of the Windows partition for data which might otherwise be on your Ubuntu /home partition. This is not a very good solution to your dilemna because it is infinitely preferable to have your data on a /home partition which is not viewable from windows. This provides a layer of privacy and security whether or not you have encrypted your Ubuntu partition.The problem is worse if your Ubuntu partition is encrypted, as any use of the windows partition will push you into use of unencrypted partition space.
So I would ask whether you have exhausted the possibility of shrinking your windows partition rather than your /home partition.That would provide you with a more viable long term strategy that won't cramp your Ubuntu /home directories and as an added bonus minimize the prospect of harming your Ubuntu partitions.
Whatever you decide, it is imperative to thoroughly back up both /home and root. This means backing up your hidden files not just the visible files.
Many partition management utilities allow for backing up and entire partition
via a "copy partition" function.
If you are interested in addressing the strategy of shrinking the windows partition please peruse the forum and post a new question if need be as I won't clutter this thread with that procedure unless you elect to go that route.
A final point is not to overlook the prospect of using the existing swap partition in some manner or even shifting other parts of the hard drive. Again, a screenshot from gparted would be very nice and helpful as well as your situation with regard to that massive windows partition which is taking up 5/6th of your hard drive!