Try running the 64-bit MinGW shell (one of the three environments provided by MSYS2), installing the package mingw-w64-x86_64-tk, and then running gitk.
mingw-w64-x86_64-tk
gitk
Gitk works on my system, and wish is provided /mingw64/bin/wish.exe.
wish
/mingw64/bin/wish.exe
I confirm C:\prgs\git\PortableGit-2.7.0-64-bit\cmd\gitk.exe works just fine.
C:\prgs\git\PortableGit-2.7.0-64-bit\cmd\gitk.exe
Make sure you are executing it from a regular CMD session or from C:\prgs\git\PortableGit-2.7.0-64-bit\git-bash.exe
C:\prgs\git\PortableGit-2.7.0-64-bit\git-bash.exe
I would not try and run it from a Cygwin session.