Well, to be consistent with the solution MS gave you about AxisBox, it should be something like:
Report(gb)[legend box(1)]<< {Label Levels( [620 640 660 680 700 725 750 775 800] ), Reverse Gradient( 1 ) }
but it doesn't work, and if you run
show properties(Report(gb)[legend box(1)])
you'll see that you have far less possibilities compared to
show properties(Report(gb)[axis box(1)])
Sorry I can't help more...