so when I try to reinstall Centos again, it prompt an error saying the packages already installed.
You shouldn't be "re-installing centos again"; just add the appropriate software.
Is there a way to change to Gnome desktop environment?
I use Package Manager and check Gnome and uncheck Kde to install and deinstall the necessary packages but to date I'm still not able to change to Gnome.
So, are you now in the situation that you cannot get into kde? What can you get into? Anything?
Did you try the chooser to choose which to get into at login? What options were there in the 'drop down' menu (if not, when you tried, what did you try and what happened)?