Filling them should lower the Q and making the absorption more broadband.
Making these sheets heavier should lower the resonance frequency.
Adding GG will also lower Q of the resonating panels, but I am not sure this is significant.
I would like to see if these resonators really contribute in any significant way to the reverberance of the room.
If they do, I would guess filling them is a way of taming them.
When I did a little investigation on plate resonators and especially 1/4 lambda resonators you could see in a waterfall when undampened they did add narrow band sound to the sound field.
When a little wool was added they didn't do anything at all. A friend of mine coincidentally performed the same measurements and reported the same results.
And I recall Eric telling the same story.
If you view life with the knowledge that there are no problems, only opportunities, you are a marketing manager.......this is my personal philosophy