If you want to install it as a local user

GNU GSRC provides an easy way to do so

Link: http://www.gnu.org/software/gsrc/

After configuration, simply specify the following commands:

cd gsrc
make -C pkg/gnu/gcc 
make -C pkg/gnu/gcc install

The second step could also be changed to speed up for an N-core system:

make -C pkg/gnu/gcc MAKE_ARGS_PARALLEL="-jN"

That executable is a "Linux executable" - that is, it's executable on any recent Linux system. You can rename the file to what you want using

rename a.out your-executable-name

or better yet, tell GCC where to put its output file using

gcc -o your-executable-name your-source-file.c

Keep in mind that before Linux systems will let you run the file, you may need to set its "executable bit":

chmod +x your-executable-name

Also remember that on Linux, the extension of the file has very little to do with what it actually is - your executable can be named something, something.out, or even something.exe, and as long as it's produced by GCC and you do chmod +x on the file, you can run it as a Linux executable.

Did you installed "ruby-dev" package?

All you need is install packages from Ubuntu repositorie (using "apt-get", "aptitude", or any other package manager front-end). No need in downloading sources. Installing package will plcase all tour required headers and libraries in standard directories, so you will not have to deal with compiler keys.

And only if you do not have required packages in repos (or it is so old), you'll have to download sourcecode tarballs and INSTALL it (accordingly included in tarball instructions, often in file INSTALL).

You should be able to build the Linux kernel with a pretty old version of GCC, probably back as far as GCC 3.2. The top-level README file in Linux 4.9.4 mentions:

 - Make sure you have at least gcc 3.2 available.
   For more information, refer to Documentation/Changes.

And Documentation/Changes gives a list of required versions of GCC (listed as "GNU C") and the other build dependencies for Linux, as well as how to check what version of each you currently have installed:

====================== ===============  ========================================
        Program        Minimal version       Command to check the version
====================== ===============  ========================================
GNU C                  3.2              gcc --version
GNU make               3.80             make --version
binutils               2.12             ld -v
util-linux             2.10o            fdformat --version
module-init-tools      0.9.10           depmod -V
e2fsprogs              1.41.4           e2fsck -V
jfsutils               1.1.3            fsck.jfs -V
reiserfsprogs          3.6.3            reiserfsck -V
xfsprogs               2.6.0            xfs_db -V
squashfs-tools         4.0              mksquashfs -version
btrfs-progs            0.18             btrfsck
pcmciautils            004              pccardctl -V
quota-tools            3.09             quota -V
PPP                    2.4.0            pppd --version
isdn4k-utils           3.1pre1          isdnctrl 2>&1|grep version
nfs-utils              1.0.5            showmount --version
procps                 3.2.0            ps --version
oprofile               0.9              oprofiled --version
udev                   081              udevd --version
grub                   0.93             grub --version || grub-install --version
mcelog                 0.6              mcelog --version
iptables               1.4.2            iptables -V
openssl & libcrypto    1.0.0            openssl version
bc                     1.06.95          bc --version
Sphinx\ [#f1]_         1.2              sphinx-build --version
====================== ===============  ========================================

.. [#f1] Sphinx is needed only to build the Kernel documentation

It's a good idea to check the Documentation/Changes file in the kernel source you're actually planning on compiling.

With all that said, it's probably a good idea to use a relatively recent version of GCC, since newer versions have better optimizers and thus can often produce faster code, and since newer versions have more readable diagnostics so it may be easier to figure out what went wrong of your build fails.

In general, I recommend using the version of GCC that is officially packaged for your system. In Ubuntu, the version of GCC that is installed as a dependency of the build-essential metapackage should always be sufficient, at least for building kernels that aren't far newer than the kernel officially packaged for your system.