the DrawRect() doesnt appear to like width and height values less than 2 for whatever reason... granted a width or height value of 1 is essentially a line. so to get around it just do a simple check to determine that the value of the roll is greater than 0.02 and it should work fine...
if roll>0.02 or roll<-0.02 then
DrawRect(gx-50,gy-100,10,roll*100)
DrawRect(gx-90,gy-100,10,0-(roll*100))
end