Gentoo-Based exGENT Linux OS Launched with Xfce 4.12.1, Linux Kernel 4.10.1 and a new bootloader

Gentoo uses a BSD ports-like system called Portage. Portage is a package management system that allows great flexibility while installing and maintaining software on a Gentoo system. It provides compile-time option support (through USE flags), conditional dependencies, pre-package installation summary, safe installation of software (through sandboxing), system profiles, and configuration file protection amongst several other features.

In fact, there is no difference between the various releases after they have been installed. Gentoo 1.4 and later are glibc-2.3.x (or higher) based. As such, running emerge --sync && emerge -uDN @world will bring the entire system up to speed with the "latest Gentoo". The differences between individual releases lie in the installation medium and pre-compiled packages. See the Gentoo Upgrading Guide for more information about profiles and their role in upgrading.

It isn't obligatory to redo every step of the installation. However, investigating the kernel and all associated steps is necessary. Suppose that Gentoo is installed on /dev/sda1 (/boot) and /dev/sda3 (/) with /dev/sda2 being the swap space.

Boot from the install CD and wait until a command prompt shows up.

ISO files must be burned in raw mode. This means the file should not just be placed on the CD, but interpreted as an entire CD.

There are lots of CD burning tools available; covering them all would be a Sisyphean problem. However, describing a few popular tools never hurts:

With EasyCD Creator select File, Record CD from CD image. Then change the Files of type to ISO image file. Then locate the ISO file and click Open. After clicking Start recording the ISO image will be burned correctly onto the CD/DVD.
With Nero Burning ROM, cancel the wizard which automatically pops up and select Burn Image from the File menu. Select the image to burn and click Open. Now click the Burn button and watch the brand new Gentoo Live CD being burnt.
With cdrecord, simply type cdrecord dev=/dev/cdrom (replace /dev/cdrom with the CDROM drive's device path) followed by the path to the ISO file.
With K3B, select Tools → CD → Burn CD Image. Then locate the ISO file within the 'Image to Burn' area. Click Start to begin the burn process.
With Mac OS X Panther, launch Disk Utility from Applications/Utilities, select Open from the Images menu, select the mounted disk image in the main window and select Burn in the Images menu.
With Mac OS X Jaguar, launch Disk Copy from Applications/Utilities, select Burn Image from the File menu, select the ISO and click the Burn button.

Yes! Probably the fastest way to do so is to install GRUB2 with sys-boot/os-prober. Read about it in the GRUB2 article and specifically about dual booting with GRUB2 here.