GSA is related to an imbalance Po4:No3, lack of Po4 essentially.
You can add 0.1 ppm of Po4 and clean the glasses every day until it stops appearing. Then, the amount of Po4 extra you added plus the Po4 you were adding is the amount your aquarium is really up taking.
You can read more about this aproach I call Method of Controlled Imbalances in English in this post at APC:
IME light is a good way of controlling it. with my recently installed MH lamp, raising the light physically so light intensities are lower coupled with added po4 eliminated GSA completely within a week.
then keeping the light at this particular height for the remainder of the tanks life worked a treat within my set up, but i am aware not everyone can raise or lower their lights. it's just an added bonus i have, just like the accelerator in a car i guess.