Yes, I know MinGW has GCC, but in my experience usually people use MinGW to compile something for Windows that was originally developed on Linux, or at least with a Linux-like development workflow. Thus it probably wouldn't be too hard to compile it for Linux if you have already compiled it with MinGW on Windows. Of course, I'm sure there are exceptions. Also, I'm not complaining, as I'm fine running Winbox with Wine. I just was surprised that it was built with GCC but is Windows-only. I think that is very unusual.
By the way, I looked at the contents of a binary I have compiled with MinGW, and it contains very similar GCC strings to winbox.exe.