gcc should be installed when you install the Developer Tools from the Apple installation disk. It should be at /usr/bin/gcc
I havn't used xcode for anything really other than light modification to system files, so I can't comment on that.
If your comfortable in Terminal, you can create your c file with vim or some editor, save it as 'whatever.c' then 'gcc whatever.c' and it should (assuming no major errors in the code) create a file called a.out which is your executable to be run. Which can be ran by typing './a.out'
gcc is actually the GNU C compiler. I do believe if you compile from the command line you will have to specify the c++ (for c++, naturally) compiler which is g++. At least, when I coded c++ in linux, I had to do this to avoid the compiler erroring out. XCode should differentiate this for you if you use it.