Don't know about GG, but with 16 levels it's pretty easy to control manually as the values are multiples of 17 (255/15 = 17). 32 levels is trickier since 255 is not even divisible by 31. But you could still make a list of all those (rounded) values and make sure all colors conform to it (0,8,16,24,32,41,49,57,65,74...)

Grafx2 has an adjustable RGB-scale. (It's a complete mode where all colors in the palette/image are forced to this value resolution)

Edited by DawnBringer - 27 September 2014 at 9:29pm

Right-click the small palette icon on the far right to open the window, then click the "/2" button three times to get to the 32-scale.