If you want to use the GNU C (gcc) compiler to compile your code from the command line, just type:
gcc myprog.c (every time you want to compile)
If you use Makefiles to compile your programs, then put the following line at the top of your Makefile:
CC=gcc
If you have problems with this compiler, send a "trouble" report with a description of the problem.
For information on gcc, see our stock answer on What C Compilers are available. You may also type the following from your prompt:
% man gcc
Finally, if you are familiar with the info system in emacs, there is information on gcc available by entering info with the keystrokes:
C-h i
After entering emacs. You will find information on how to run the GNU compiler.