ext partitions are formatted by default with a number of reserved blocks set aside that can only be written to by root. As the mke2fs man page explains:
Specify the percentage of the filesystem blocks reserved for the
super-user. This avoids fragmentation, and allows root-owned
daemons, such as syslogd(8), to continue to function correctly
after non-privileged processes are prevented from writing to the
filesystem. The default percentage is 5%.
You'll note that 5% is exactly the amount of unoccupied space as shown by gparted.
You can view the reserved block count by running (as root)
tune2fs -l /dev/sda5, and adjust it with
tune2fs -m <percentage> /dev/sda5. However, I would strongly recommend against it.
As the man page explains, the reserved count is not only used to give root processes some "wiggle room" (critical on system partitions), but to reduce fragmentation (especially critical on /home partitions). That 5% buffer provides the filesystem with spare blocks it can use to write files contiguously, rather than having to fragment them into available space scattered all across the disk.
(The reserved block count is a percentage, rather than a list or range of blocks, because the filesystem doesn't care specifically which blocks are reserved. It can juggle free blocks around as needed, as long as a certain percentage are kept reserved.)
Forcing the filesystem to burn up that last 5% of space will give you a paltry amount of extra storage space, in exchange for sending the fragmentation of your filesystem skyrocketing and degrading performance. If you must reduce the reserved block percentage to gain some space, consider setting it to 4% or 3% rather than 0.