Transparency not saved on OSX

Hello all,

I’m trying to create a transparent gradient image as I would in ImageMagick in GraphicsMagick. The following command works for me in IM:
convert -size 1280x720 -define gradient:direction=SouthWest gradient:'rgba(0,255,0,0.0)'-'rgba(0,255,0,1.0)' im.png
and generates the attached output.

However, if I run the same command in GM:
                gm convert -size 1280x720 -define gradient:direction=SouthWest gradient:'rgba(0,255,0,0.0)'-'rgba(0,255,0,1.0)' gm.png
I get the attached output. As you can see, it is a solid color.

I have GraphicsMagick installed at a quantum depth of 16 and have tried with 32:
➜  ~ /usr/local/bin/gm version
GraphicsMagick 1.3.25 2016-09-05 Q16 http://www.GraphicsMagick.org/
Copyright (C) 2002-2016 GraphicsMagick Group.
Additional copyrights and licenses apply to this software.
See http://www.GraphicsMagick.org/www/Copyright.html for details.

Feature Support:
  Native Thread Safe       yes
  Large Files (> 32 bit)   yes
  Large Memory (> 32 bit)  yes
  BZIP                     yes
  DPS                      no
  FlashPix                 no
  FreeType                 yes
  Ghostscript (Library)    no
  JBIG                     no
  JPEG-2000                no
  JPEG                     yes
  Little CMS               no
  Loadable Modules         yes
  OpenMP                   no
  PNG                      yes
  TIFF                     yes
  TRIO                     no
  UMEM                     no
  WebP                     no
  WMF                      no
  X11                      no
  XML                      yes
  ZLIB                     yes

Host type: x86_64-apple-darwin15.6.0

Configured using the command:
  ./configure  '--prefix=/usr/local/Cellar/graphicsmagick/1.3.25' '--disable-dependency-tracking' '--enable-shared' '--disable-static' '--with-modules' '--without-lzma' '--disable-openmp' '--with-quantum-depth=16' '--without-gslib' '--with-gs-font-dir=/usr/local/share/ghostscript/fonts' '--without-x' '--without-lcms2' 'CC=clang' 'CXX=clang++'

Final Build Parameters:
  CC       = clang
  CFLAGS   = -g -O2 -Wall -D_THREAD_SAFE
  CPPFLAGS = -I/usr/local/Cellar/freetype/2.6.5/include/freetype2 -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/libxml2
  CXX      = clang++
  CXXFLAGS = -D_THREAD_SAFE
  LDFLAGS  = -L/usr/local/Cellar/freetype/2.6.5/lib -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/lib
  LIBS     = -lfreetype -lbz2 -lz -lltdl -lm –lpthread

Doing an identify on the 2 PNGs tells me that im.png is “true color with transparency” and gm.png is just “true color”. Any idea what could be going wrong here?
Thanks,
Anubhaw Arya

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