Ok, so firstly I'm using Windows 10/Ubuntu 16.04 LTS. Before anyone asks, the unallocated space is not by the Ubuntu partition, it is by the Windows partition, which makes this more difficult. I had looked at doing this while in Windows, but Windows disk manager doesn't show EXT4, and EXT2 volume manager doesn't show unallocated space so...
I'll try and get a screenshot of GParted within 30 minutes, if that helps. Thanks.
Possibly relevant, but the Windows partition lists the error "Unable to read the contents of this file system!" Any idea about that?
Edit 2: Another update! Partial success so far, after shrinking the Windows partition I gained about 40GB, which I merged with the Ubuntu partition. There's still 12GB by the Windows, but unfortunately I keep getting an error whole trying to move Windows. I'll figure that out soon. :) Thanks!