Actually GCC for Windows target is like pain in the ass. Look for how many problems ReactOS had with it. Also this would need SEH to be implemented. Clang gives that out of the box.
"Don't look at ReactOS code either (not even header files). A lot of it was reverse-engineered using methods that are not appropriate for Wine, and it's therefore not a usable source of information for us. [1]"
Why don’t you help ReactOS?
ExectOS goals are very different from ReactOS, and contrast the project’s core philosophy as being quite on different paths. While ReactOS aims to replicate Windows® NT™, ExectOS is a completely new Operating System implementing the XT architecture which derives from NT™. Although both projects share the goal of being NT™ compatible, they intend to achieve it in different ways. What ReactOS tries to replicate, ExectOS only implements as a compatibility layer. Thanks to that, ExectOS does not need to strictly follow NT™ architecture and is capable of providing modern features.
Do you intend to cooperate with ReactOS to achieve common goals?
No. We share Wine’s opinion on the inappropriate reverse-engineering methods used in the ReactOS project, as well as its association with the TinyKrnl project, which used every possible method of achieving the end result of having a 100% compatible results. This especially applies to the so-called ‘dirty’ way.