It seems GParted mangled something with the partition structure that caused my Windows 10 install to no longer boot after resizing its partition. This probably had something to do with the Hybrid MBR system that Bootcamp uses.
I tried a lot of stuff and frankly these instructions may not actually fix your problem. I believe the solution here was to move away from Hybrid MBR and then rebuild the EFI/BCD data for Windows.
Convert the EFI/MBR partition to standard EFI within macOS. Instructions are in this article about installing Windows 10 without Bootcamp, under “Ensuring that a Hybrid MBR is not used”.
Use a Windows 10 install USB stick to bring up a command prompt. Assign a drive letter to the EFI volume via the following commands:
diskpart
select disk 0
list volumes
select volume [x] #whichever one says EFI
assign LETTER=Y
If you can’t see the EFI volume and assign it a letter, then something is “wrong” with the EFI partition, so Windows is not able to see it. Why that might be the case on your system after following the instructions in the other article, is an exercise left up to you to figure out.
After the EFI volume is mounted to drive letter “Y:”, run:
bcdboot C:\Windows /l en-us /s y: /f ALL
bootrec /rebuildbcd
bootrec /fixboot