GCC builds of AviSynth+ require a bit different setup than standard MSVC builds of AviSynth+.

AviSynth.dll still goes in the normal installation path you use for AviSynth+ (typically,
System32/SysWOW64).

However, GCC-built C++ plugins cannot reside in the same directories as MSVC-built C++ plugins,
and AviSynth+GCC cannot use MSVC-built C++ plugins.  Mixing C++ plugins built by MSVC and GCC
in a single directory will cause AviSynth+ (MSVC or GCC) to crash.

For safety, AviSynth+GCC ignores the plugindir2_5 or plugindir+ registry entries, so any plugins in
your normal installation won't cause issues for it.  It has its own registry entry, plugindir+gcc (which
AviSynth+MSVC ignores, correspondingly), that can be used to look for plugins, like this snippet shows:

[HKEY_LOCAL_MACHINE\SOFTWARE\AviSynth]
@="C:\\Program Files (x86)\\AviSynth+"
"plugindir+gcc"="C:\\Program Files (x86)\\AviSynth+\\plugins64_gcc"

C plugins are unaffected by this 'quirk' and can live alongside either MSVC-built or GCC-built C++ plugins.

As far as programs like FFmpeg or x264 go, 64-bit builds that normally expect an MSVC build of AviSynth+
can see or use 64-bit AviSynth+GCC.  Regular 32-bit builds cannot use 32-bit AviSynth+GCC due to calling
convention differences between MSVC and GCC.  

32-bit programs wanting to support AviSynth+GCC need to use -DAVSC_WIN32_GCC32 in their CFLAGS
to switch ON support for AviSynth+GCC and switch OFF support for 32-bit MSVC builds of AviSynth+.