To use unallocated space in Linux:
Use GParted to increase the size of your Linux #partition (thereby consuming the unallocated space.
Run the command resize2fs /dev/sda5 to increase the file system size of the resized partition to its possible maximum.
Reboot and you should have more free space on your Linux file system.
More Info > https://linuxapt.com/blog/fix-unallocated-disk-space-to-extend-a-disk-partition-in-ubuntu-debian-rhel-and-centos