Hello dear GNU GCC team! I would like to ask you some questions:
1. Is GNU GCC completely free software? Can I use it for commercial and
non-commercial projects without opening the source code?
2. Can I use GNU GCC on windows, linux and macOS? And at the same time not
to use additional software, for example: Visual Studio?
3. Under what circumstances should I make my projects open source?
4. Can I use any headers in my projects? Or I should my projects open
source?

P.S. I really like your GNU GCC product and would really like to use it for
C/C++ practice, maybe use it for commercial products in the future. But I
really don't want to make my projects open source :)