I recently installed CentOS 6.2 in VirtualBox 4.1.12 (because it's free). During the boot from the liveDVD, I chose to "install" not "install (Text)". During the install I chose use entire disk, gave it a hostname, and a root password, and selected my timezone. After a pleasant few minutes watching the status bar fill, and waiting for the post-install scripts and the boot loader to run, I was presented with the congratulations, reboot screen.
On the first boot, (after a hurry to get the CD/DVD disconnected) I had to accept the user license and create a user account. Kdump complained about not having enough memory, but firstboot continued and brought me to the gdm login screen. If you press enter, or click on YOUR user name that was created at the beginning of firstboot, the bottom of the screen will also offer a selection/drop-down of Gnome/KDE as your session. This will also become your default session.
In my case, I chose KDE, but in reality, I prefer Gnome with the exception of my CD/DVD applications and my terminal. I just can't live without the old KDE Konsole (not krazy about the new one though). I like konquerer and kopete, but prefer dolphin and pidgin. Evolution is too much like outlook for my taste, I use thunderbird and firefox for familiarity between environments. I run linux to get as far from microsoft apps as possible.
To answer your question, you can make your centos boot into graphical mode by modifying /etc/inittab:
As for Gnome vs KDE, selecting a desktop session manager during login should solve the problem. Otherwise, in gnome, under system>preferences>preferred_applications, you can set your browser, mail reader, media player, terminal, etc. I just can't find and don't remember what the magic used to be for switching desktops in the past. It used to be possible to just change on the fly.
I hope this information was helpful,