Hi Dariusz,
What I do is unzip the GCC to @unixroot/usr so using GCC 9.4.0 as an example, I have w:\usr\local940. Then in @unixroot/usr/bin I have the attached gcc940env.cmd and run that in the command window where I want to use the new GCC. Line 74 is where you point to the local940 or whichever GCC,
call GCC322plus sPath, 'gcc940', 0, sLinker;
Adjust the gcc940 line to whichever GCC you're using, I have multiple gcc*env.cmd files for different GCC versions.
Should really double check that this includes all the C++ stuff, KOMH originally did the cmd file quite a while back.