Did you install ghostscript at the same time as Imagemagick (in the same install command) from Homebrew? Not as two separate installs.
Find your delegates.xml file in your installed system.
Then open in a text editor and modify the lines where it says gs to be the full path2/gs. I do not know where that is in your system. So you may have to find it.
In unix, I can do (assuming my install puts this file in /usr):
/usr/local/etc/ImageMagick-6/delegates.xml
I do not know where Homebrew puts its install of Imagemagick.
<delegate decode="ps:alpha" stealth="True" command=""
gs" -sstdout=%%stderr -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 "-sDEVICE=pngalpha" -dTextAlphaBits=%u -dGraphicsAlphaBits=%u "-r%s" %s "-sOutputFile=%s" "-f%s" "-f%s""/>
<delegate decode="ps:cmyk" stealth="True" command=""
gs" -sstdout=%%stderr -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 "-sDEVICE=pamcmyk32" -dTextAlphaBits=%u -dGraphicsAlphaBits=%u "-r%s" %s "-sOutputFile=%s" "-f%s" "-f%s""/>
<delegate decode="ps:color" stealth="True" command=""
gs" -sstdout=%%stderr -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 "-sDEVICE=pnmraw" -dTextAlphaBits=%u -dGraphicsAlphaBits=%u "-r%s" %s "-sOutputFile=%s" "-f%s" "-f%s""/>