Gparted, that use mkfs.ext4 to create a ext4 filesystem, reserves 5% of the space for super-user, as explained in man page:
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%.
Now, 5% of about 300GB is just the 15GB you see already used.
More interesting, you can avoid this if you feel do not need it, simply creating the filesystem with, for example,
sudo mkfs.ext4 -m 0 /dev/sda1
where 0 means 0%.
I think Gparted see more space because is run by super-user, so part of that 15GB are free for him. Nautilus is run by the user, so the space is less.