That's right low CO2 and PO4 levels are often thought to encourage GSA, but ultimately it's a high light issue and a question of balance. I had some growing on my aquarium glass and some on plants in high light areas, but I usually don't use mega watts of lighting over my scapes. Compared to a lot of folks my lighting is of fairly moderate intensity. But I never really struggle growing plants, even carpet plants grow low and compact. I think good CO2 flow and distribution is of greater importance.
The lack of GSA on my epiphytes is also more to do with where I place them in the scape. Anubias definitely needs to be placed in the shade. It's relatively slow growing even in good light and that means the leaves are prone to algae. Some Buces usually do well in fairly high light since they grow relatively quickly so new leaves shade out the old before they can attract algae. The leaves also develop a beautiful iridescent sheen in higher light.