If Gparted isn't letting you resize it, don't try and force it to resize with something else, you'll fuck it up. I did that once when I dual booted, and since Gparted wouldn't let me give Ubuntu more space, I used some Windows program and forced it to, and I couldn't boot into GRUB anymore.

When you installed Ubuntu, did you choose the "install side-by-side" option, or did you manually set the partitions?