Is it safe to resize Windows partition with GParted?

You can resize the Windows partition, and it will still boot. Windows will give an error at start-up the first time saying the disk size doesn't match it's records, it will run checkdisk and update it's records, and then work fine. I've done this many times.

You cannot move the Windows partition though or it won't boot. It's worth pointing out that if you shrink from the left side of the Windows partition what you are actually doing is shrinking, and then moving; ONLY shrink from the right side of the partition. You also should not expand the partition on the left side as again you're actually moving it to the left, and then expanding off the end; ONLY expand from the right side of the paritition.

I would also point out that partitioning is never a 100% safe activity. You are altering the partition table, if that fails (slim chance) your entire disk is at risk for data loss. It's always a good idea to have a backup of your entire drive before engaging in any form of partitioning, not that most people actually do it.

Windows can shrink its partitions, too, so why not go with that?

Simply open Disk Management, right-click on the partition you want to resize and fire away. Safety guaranteed.

If you find the space you can gain to be too small, you could defragment the partition (even if it's on a SSD). Don't expect too much, though, there are many immovable elements on a partition.