Since GSA grows mostly on glass, rarely on plants, it would be more accurate I would think to assume it was a mix of CO2 and PO4 that prevents new germination of the GSA spores.
Competition has nothing to do with it.
This is rather obvious in terms of PO4, NO3 etc..........or light..........but............not CO2 very often.
Light and nutrients are easy to simply add excess non limiting resources.........this is a control for resource competition.
Many aquarist seem to have poor abilities to do the basics of competition studies for aquatic plants, but this is VERY basic logic in plant ecology and applies to algae as well.
We can verify the light and nutrients easily.
CO2 is much moire troublesome for most planted hobbyist.
Much harder to really verify, but..........you have 2 of the 3 main things, so take care of those, then focus on CO2 thereafter.
Use algae and good plant health as the test kit to slowly add more CO2.
CO2 is the most limiting nutrient for plants, and virtually never limiting for algae.
So it is really about hurting your plants and their growth, not algae, they just respond to poor plant health when there are plants present.
Nutrients/light otherwise when there are no plants.