With traditional SAS/Graph procs (such as gchart), you can use a legend statement, and specify mode=share and/or specify an offset= to move the legend from the default location.
For example, you could use a blank footnote statement to add the desired amount of white-space at the bottom of the page:
footnote1 height=5pct " ";
And then a legend statement something like the following untested pseudo-code...
legend1 mode=share position=(bottom middle) offset=(0,-5);