You can then follow the standard compilation instructions provided above to compile Gmsh and its dependencies.
You can then follow the standard compilation instructions provided above to compile Gmsh and its dependencies.
Note that the application binary interface (ABI) of mingw64_x86_64-gcc-g++ is different than the ABI of Miscrosoft Visual C++. So you won't be able to e.g. use the precompiled OpenCASCADE Windows DLLs: you will need to recompile OpenCASCADE with mingw64_x86_64-gcc-g++. If you want to use these DLLs, you will need to also compile Gmsh with Visual C++.
Note that the application binary interface (ABI) of mingw64_x86_64-gcc-g++ is different from the ABI of Miscrosoft Visual C++. So you won't be able to use e.g. the precompiled OpenCASCADE Windows DLLs (as they are compiled with Visual C++): you will need to recompile OpenCASCADE with mingw64_x86_64-gcc-g++. If you want to use the DLLs compiled with Visual C++, you will also need to compile Gmsh (and FLTK) with Visual C++.