i don't thiunk anybody gave the gmerlin plugin a try on w32/mingw yet (though e.g. vlc is known to work)
the usual procedure is to tell Gem's configure where to find gmerlin (PKG_GMERLIN_AVDEC_CFLAGS and PKG_GMERLIN_AVDEC_LIBS) and hope it will autodetec everything....
gmerlin is detected but I couldn't make it because of this:
'glutMainLoopEvent' was not declared in this scope
...
./configure --without-all
compiles gem.dll but couldn't create some other library files
/filmAVI.cpp:230: undefined reference to `AVIStreamRead@28'
...
I haven't tried yet with libvlc