The Small Business Edition of an Embedded Wizard Platform Package is provided as libraries. In case of NXP or STM32 targets, these libraries are created with GCC, IAR and Keil.
In case of Professional Edition, the entire code of Graphics Engine and Runtime Environment is provided as source code. The source code can be used with any toolchain.
We have not tested if GHS toolchain is able to work with GCC, IAR or Keil libraries. Maybe you can ask the GHS support.