I am not aware of any person that build GNU Pascal on Windows in 64-bit mode.
I tried, but failed. I needed substantial help from Dave Bryan just to get the 32-bit compile to work. So, on Windows 64-bit machines I use the 32-bit MinGW, and that's fine.
The 64-bit MinGW seems to be a significant diversion from the 32-bit, but in theory the compile is of course possible. I have a student this summer who I will apply to the project, but it may be too much for him. In the long run, I'm hoping to have one of my people figure out how to compile GPC with a newer version of GCC, which I think will make the 64-bit compile easier. I understand that this update would require a lot of work, but I don't have any clear idea of what the work is. So my first step will be to have my summer student figure out what has to be done.
Yours, Kevan