Why is there 4.86(maybe 15.1) GB of USED-space on a newly partitioned and formatted 298 GB drive (as ext4 by Gparted)

Gparted, that use mkfs.ext4 to create a ext4 filesystem, reserves 5% of the space for super-user, as explained in man page:

-m reserved-blocks-percentage
   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.

There are different ways of counting space on a filesystem ranging from bytes not in use to free clusters, etc. In the absence of better information, I'd take gparted's report as more "accurate".

Filesystems require overhead for i-tables, free lists, superblocks, superblock backups, and - in ext4 - journaling. Your reported 4.86 out of 298GB is a whopping 1.6% overhead in order to keep track of your stuffs; that strikes me as pretty low overhead.

Aside from the space reserved for root, see my other answer here. The short of it is that the ext series of filesystems reserves space for the inode tables at format time, which with the default settings, amounts to 2 mb for every 128 mb of disk space.