My limited experience with it was not with a riding horse, so take this with a grain or two of salt. I had an old broodmare with a trashed stifle who benefited from gabapentin. It didn’t totally take her lameness away, but I tried taking her off of it once and it was very evident that it was helping.
It’s super cheap (relatively speaking) in plain tablet form. I didn’t have it compounded. I was feeding 4800 mg per day, split in two feedings. I ground 3 starlite peppermints with the tabs in a coffee grinder, drizzled some EO-3 oil over her feed, sprinkled the powder over the top and gave it a good shake. She was a picky eater and as long as I put enough peppermints in, she was content with eating it. She never looked stoned or anything.
I gave it a try for a very short time on a younger mare who has a healed broken pelvis. I didn’t notice any difference in her unsoundness, but she looked dumpy. I know it can cause nausea in humans. No idea if horses get nauseous, but that’s what she looked like. So I took her off and that look subsided pretty quickly. Maybe that’s just what she looks like when she’s high haha.
None of this may apply to you, or some of it may. But perhaps it’s helpful for someone. I fed it for several years, but just don’t have experience with giving it to a horse under saddle.