As far as I know, GParted can’t deal with LVM yet. Unless anyone else knows of a graphical LVM front-end, you’re stuck with terminal commands. If the install is only a couple of days old, it may be easiest just to reinstall…
Assuming that you need to claim the space from /home, the commands are basically:
- Shrink the /home file-system (resize2fs) to free up more space than you need;
- Shrink the /home logical volume (lvresize) to free up space;
- Grow the /home file-system into its newly-sized volume (resize2fs without specifying a new size);
- Grow the / logical volume (lvresize);
- Grow the / file-system (resize2fs without specifying a new size).
The reason for (3) is that getting the sizes exactly right can be difficult, so it’s safer when shrinking a file-system to over-shrink it, then grow it back into the reduced volume.
Give / 10–20 GiB, depending on how much extra software you normally install.