I've only combined benzos, gabapentin, alcohol, phenibut, and floribut to psilocybin containing fungi as far as downers go, but I always had to go to 1.25x/1.5x the dose I wanted to feel as far as fungi, or use .5x/.75x the amount of the downer when mixing them, to land within the range of my expected effects.
I used to think it related to GABA antics that relate to the serotonin syndrome, but isn't gabapentin just a channel blocker? I wonder if that means that the effect we're observing here relates more to the net impacts of these two kinds of drugs on how we perceive drug effects overall, I'm not sure but it's quite interesting.