I can think of many ways to try but the most direct one would be the following.

Dissolve calcium sulphate in an acid and filter glass fibers, weigh them and estimate volumic ratio using GF weight vs. total wieght correcting via the densities of gypsum and a sodo-calcic glass (most common glass for glass fibers) ? This should be enough to give you a correct estimation.

Dear Katia, could you determine the use of this mixture? Is it prepared for experimental scale, or it will be used as surface-coating material at building shell? In my opinion the end use will determine the proper preparation of the mixture. At large scale exposure to outdoor weather conditions, the following criteria should be met:

- thermal insulating properties, in alignment with thermal bridging insulation of the building shell

- waterproofing capabilities and suitable hydric resistance, which are linked to porosity of material against water channeling through pores due to water precipitation and humidity

- resistance to aging of material due to solar radiation, temperature fluctuation, and windy conditions

- acceptable physical and mechanical properties against fatigue and fracture

- economic profile/co-evaluation, in terms of local availability of the selected materials, the building cost and the payback period considered, regarding expenses for energy consumption as well as operating & maintenance (O&M) costs

Of course the aforementioned criteria should be also considered as testing parameters for material samples at an experimental scale.

You may want to check:

https://www.researchgate.net/post/How-might-I-determine-glass-fiber-content-in-composite