Unallocated space at the end of HDDs

Extend /dev/sda4 to the end of the disk. You can only have up to 4 'primary partitions' on this disk. Once you expand it, you can create an NTFS partition (a Logical partition) at the end inside of /dev/sda4

The secondary label at the end of the volume is required by GPT; a disk without it is corrupted and may not be recognized by all systems. Using a corrupted format in order to gain a tiny amount of space, on a disk that's probably a few hundred gigabytes at least, doesn't make much sense.

However, the actual GPT label is only about 68 kilobytes in size. Modern partitioning software typically aligns the start of each partition on a 1MB boundary for performance reasons, since that's a multiple of any realistic disk sector or flash erase-block size, but there's no real need for the end of the last partition to be on a 1MB boundary. GParted may do that by default, but gdisk lets you create a partition that goes right up to the last usable sector (68k from the end of the disk).

Note, however, that the 1MB alignment of the beginning of your partitions means that you also have about 1MB of unused space at the beginning of the disk, between the GPT label and the first partition. If you know your disk's physical sector or erase-block size, you could align your partitions to that instead of using 1MB boundaries, in order to move the beginning of the first partition closer to the beginning of the disk. (But this seems like more trouble than it's worth.)