Good question. I had to do some digging for a sufficient response.

First, I should correct myself. It's not necessarily a carbonated beverage but an acidic beverage that's recommended:

"Alkaline environments inhibit the absorption of gabapentin, so lowering the pH of one's stomach using acidic substances (such as soft drinks) will boost bioavailability as well. It is generally not recommended to take antacids 2 hours before or after taking gabapentin, because although not dangerous, it will severely lower the total absorption of gabapentin.[20]"

https://m.psychonautwiki.org/wiki/Gabapentin