Formatting HDD: unused space ?

I've put an internal HDD into a box in order to make it an external one.
I used Gparted to format it to ext4. so far, so good.
But Gparted announces 232GiB, when Thunar, and the internal case says 250.
I know there's always a bit of a difference between the announced number and the reality,
but this seems unusually huge.

Also, how do I encrypt an external drive?
I heard of Cryptsetup, but is there a preferred way (most secure, simplest)?

Last, about programs needing root:
I use sudo -i instead of gksudo,
but the CLI that remains is annoying.
I know it has it's use to debug problems,
but is there a way to close it without closing the program?
I use i3, so I could always go full screen easily.
But maybe there's a simpler way.

Thanks!