CodeWarrior 10.5 uses Freescale in house CW compiler (ARM compiler as you see in Project Properties). This compiler is Freescale proprietary. Whereas gcc is open source GNU C Compiler.
MCU10.5 has both Freescale CW and GCC v4.6.2 build tools. User can select tools of his choice. You will see there's an option to choose compiler when creating a new bareboard project for Kinetis.
I attached two documents, one is to explain why we add GCC compiler from CW10.3, the other is to show how to porting a project to GCC compiler. Hope they could help!