You can workaround this by changing the GnuPlot libfreetype library to point at the system one:
# Change to your GnuPlot install directory...$ cd /Applications/Gnuplot.app/Contents/Resources/lib# Backup the provided libfreetype...$ mv libfreetype.6.dylib libfreetype.6.dylib.bak# Link to the X11 provided one...$ ln -s /usr/X11/lib/libfreetype.6.dylib .