As far as I am aware, gparted is not really the right tool for this. As mentioned in the thread linked, you might want to look at the tools for manipulating LVMs here, like lvresize
.
The problem is that the pve-data thin pool can not be shrinked. Therefore, if you want to assign some more space to the root lvm you would need to backup or migrate all your VM disks somewhere else, delete the thin-pool, increase the size of the root partition and subsequently recreate the pve-data partition.
However, depending on what you want to use the storage for, you don't really need to do that. You could also create another LVM within the thin-pool and mount it somewhere on your root partition. Especially in cases where you only have a couple of big files, or are in complete control of where they will be written to, this should be a work-around without much hassle.
PS: The thread you linked talks about an installation on an SD card, if you are doing the same, please note that this is not a recommended setup at all. Proxmox installations on flash media like USB sticks and SD cards usually start failing after just a few weeks due to its frequent write operations and extensive logging.