GParted does not allow creation of mount points. It does allow setting certain flags, but that’s not the same thing. I’m uncertain that GParted is installed by default on Manjaro KDE. Perhaps you are confusing it with KDE Partition Manager.
If you need to change the mount point of an internal volume, you can do so in /etc/fstab, for example. See:
- fstab in the Manjaro Wiki
- How to mount a partition using fstab
In some cases this might best be done in a chroot environment manjaro-chroot -a from a bootable Live Installer.
I hope this helps. Cheers.