xdg is your friend! Let me find a link…
code:
export DISPLAY=:0
xwd -root -out screenshot.xwd
Convert screenshot.xwd with ImageMagick’s “convert” or, open in Gimp and export that way (I use the gimp method).
I like this method because it’s already installed, and doesn’t require a ton of bloated libs be installed on your GS. Could also try the x112vnc stuff, but I had mixed results with that.