Gparted hast nothing to do with this.
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The owner of a file on a ext4 filesystem is set, when the file is created.
So create any directory / file on it, then change the owner to yourself. -
The owner of the “root=/” of your ext4 filesystem is the owner of its mountpoint
So change the ownership of the mountpoint to yourself
There is a [HowTo] about this, but i did not find it yet.