Cgo: sorry, unimplemented: 64-bit mode not compiled in

Downloading the latest TDM-GCC-64 release worked for me! Make sure to add C:\TDM-GCC-64\bin to PATH Environmental Variable.

1. Short Answer:
   I tried many versions, the Only working version for both 32-bit and 64-bit go build in 64-bit Windows OS is tdm64-gcc-5.1.0-2.exe (see the tdm-gcc page).
   You may install it to C:\TDM-GCC-64\ and add C:\TDM-GCC-64\bin to your PATH (on top = or left).

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2. You may try MinGW-w64 - for 32 and 64-bit Windows Or x64-4.8.1-release-posix-seh-rev5.
   This versions works fine just for the 64-bit build (not 32-bit target on 64-bit OS).

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3. For Windows OS run your command prompt: cmd (terminal in Linux) then run this command (whereis gcc in Linux):

   where gcc
   

   Then if there are multiple gcc paths, then you may edit the order of these and put the one you need in the top or remove all others.

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4. See:
   • Differences between Mingw-w64 and TDM-GCC for a simple GDI project
   • What is the difference between Orwell's MinGW and TDM Dev-C++ versions?

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I hope this helps.