a limitation no more
I believe that Gandolf will mean that he and Gandalf will discover that it is futile to try to create only good, or have even a preponderance of good, and they will abandon the attempt.
This will mean that the brothers will reach an understanding with the nature of the universe, and cease to resist it.
This may also mean that the two will begin to operate on a higher level, at the state or context prior to good and evil, that contains both. This may also mean their abandonment to superimpose their will, whether to create good, or as others, to create evil, over the will of the universe. This will put them in balance and in harmony with supreme forces of the infinite.