Centos is a professional Linux, not a "toy" Linux. Everything is not done for you.
A lot of people who are new to Linux get confused about the word "Free". They think "free" is like "free beer". "Free" means "liberty". When you use a distribution were everything is done for you, all the "liberty" is lost. Centos comes with "liberty", and it will not do everything for you.
Now that I am done with my "soapbox".... There is a software manager in the Centos distribution of GNOME but I never use it. It is nothing but a graphic shell with a list of apps. It makes use of rpm, and yum. You can get to it my clicking on Applications then scrolling down to Add/Remove software.