With Kamilion's help on the Atari IRC getting this to work was dead simple.
I have a Atari RGB to VGA connector.
I just plugged it into the GBS's VGA port and made this little adjustment
A hard drive block jumper which bridges the hsync and vsync on pins 3 and 4 of the 8 pin video connector.
I have an STE with a modulator, so I'm not really sure if this works on every machine. But it worked without a hitch on mine!