GCC is not in Nim, mingw binaries are just provided on the Nim website. You can use MSVC or some other compiler through the configuration.
The only real antivirus problem I had with Nim is when one of my friends couldn't start a win64 binary I built. However when I gave them the win32 version it worked. But there is genuinely 0 reason to think that Nim itself is malicious. Build it for yourself if you want, if that still doesn't work, change a few compilation options. I posted a comment on the github issue about my theory for why this is happening, in my opinion it explains a lot of what is happening:
