Hi Michael,
I just pushed build fixes to wine-gecko-2.47 branch:
https://sourceforge.net/p/wine/wine-gecko/ci/master/tree/
I have more plans for the release and it still needs more testing, but I was able to build with Git mingw-w64 and GCC 9.2 (still with python 2) from that branch. Please let me know if it works for you. Ideally, no additional tweaks should be needed. If you still need to patch Wine Gecko, I'd be interested to hear about it and hopefully have it fixed in Wine Gecko git.
Jacek