I can't remember exactly, as it's been a few years. I'm pretty sure gparted would have been one of the first things I looked into, though.

Looking at my disk in gparted now, while it shows the LVM physical volume (and would presumably allow resizing it if it weren't in use), it doesn't seem to show the logical volumes and their filesystems inside of the physical volume at all. The partition information window for the LVM PV does list the logical volumes, but not the main window where you can plan the changes.

I don't know if that's a general limitation of gparted or if it's a peculiarity with my setup, but that's how it appears to me.