Hello, I have never used a RTOS but can see some benefits to its use. As I don’t have any experience, I ask the following question: Is it difficult to use the existing GCC port with the ICCAVR compiler? Thanks for any assist.

You would have to generate your own ICCAVR port.c and portmacro.h files.  You can use the existing GCC and IAR ports to see how this is done.  The syntax in these files is specific to the compiler (hence they are in the portable directory) but the implementation details are the same.