Mostly hg8 has provided good answers; however....
How is it possible that I have 1.02 MiB of space unallocated? Is it necessary to keep it?
The answer to which hg8 has linked on this one is poor. In order to optimize performance on a variety of disks, most modern partitioning tools align partition start points to 1 MiB values by default. (See this article I wrote for technical details and some benchmarks for one particular disk technology.) Thus, there will be a close-to-1MiB gap at the start of the disk, since there's a small bit of space used at the start for partitioning data structures. GParted hides this gap because it's unimportant.
Many tools, including libparted (used by GParted,
parted, and the Ubuntu installer) also align the ends of partitions. This isn't necessary for optimizing the performance of the partition being created, but it enables the next partition to be properly aligned without a gap. One consequence, though, is that there's often about 1 MiB of unused space at the end of the disk. In your case, it's a tiny bit more than 1 MiB (I'm guessing because a rounding error caused it to be too big, although that's just a guess), and GParted is showing it, unlike the gap at the start, which is hidden.
Small ~1 MiB gaps betwee partitions, or at the start or end of the disk, are no big deal. Your disk in particular is 465.76 GiB in size, so that 1.02 MiB gap is (1.02 / (465.76 * 1024)), or 0.00021%, of your disk's capacity. It's trivial. You can certainly extend the partition to fill that gap by using another tool (like
gdisk -- you'd delete the existing partition and create a new one with the same start point but an end point a little further out); but then to use the space you'd need to extend the filesystem it contains, and the filesystem's own data structures might not even make use of the extra space. Furthermore, mucking about with partitions in this way carries some danger, so you'd be risking your entire OS installation to get 0.00021% more disk space. If you're that desperate for disk space, you need to buy another disk, not waste time trying to recover enough disk space to hold about 1/600th of an Ubuntu
So: Don't worry about it.