FWIW, the western PCHA (California affiliate of USEF) won’t use a facility that has only the GGT-type footing. I thought it was something about it being too grabby for slides and spins. A local facility had to remove the fancy footing from one ring to accommodate the western shows.
However, the only reference to synthetic footing I can find in their rule book is to a trail class that is a finals sort of thing:
“The PCHA Trail Classic is not to be held on synthetic footing. PCHA Trail Classic must be competed on an ordinary dirt/sand mixture footing. This rule was passed to start in the year 2013 for the Classic (in August 2013). This rule holds true for any subsequent PCHA Trail Classics.”