question on installing gnat on linux mint

["Nasser M. Abbasi" <nma@12000.org>]

Basic question:

I just installed Linux mint 18.2 which comes with gcc 5.4

>gcc --version
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

To install Ada, I saw the command to use is

sudo aptitude install gnat

When I try the above, it says

------------------------------------
The following NEW packages will be installed:
   cpp-4.9{a} gcc-4.9{a} gcc-4.9-base{a} gnat gnat-4.9{a}
   gnat-4.9-base{a} libasan1{a} libgcc-4.9-dev{a}
   libgnat-4.9{a} libgnatprj4.9{a} libgnatvsn4.9{a}

The following packages are RECOMMENDED but will NOT be installed:
   ada-reference-manual-2012 gnat-gps
0 packages upgraded, 11 newly installed, 0 to remove and 229 not upgraded.
Need to get 24.4 MB of archives. After unpacking 101 MB will be used.
Do you want to continue? [Y/n/?] n
---------------------------------------------

My question is, why does it want to install gcc-4.9
when Linux comes with gcc 5.4 on it? Does this mean gnat 4.9
is build with earlier gcc? Will installing gnat change/overwrites
the version of gcc on my system? or does it install its
own gcc 4.9 somewhere else?

Thanks
--Nasser