I have some gabapentin 300mg which expired in 2009?

If, they were maybe two years old I would say they could probably be OK. Drug manufacturers purposely make medications to expire so they make money. Most medications have a two year shelf life and are also at the pharmacy for a little while and the manufacture of themselves. The rule of thumb that I have been taught if it’s expired more than 3 to 6 months after the X expiration date... depending on the medication & it’s packaging if, it’s in a blister pack they have the actual date on them they would expire. Ten years is way too old to even, chance using it. Compounds change in medication either becomes weaker or stronger or change altogether.
I took two-year-old blood pressure medication I had as I ran out of them I had very bad reactions to in November. I took Aleve last week that I didn’t realize were expired in 2017 and they just didn’t work.
I would either take them to the pharmacy for medicine disposal or if you have a cat putting them in cat litter or even coffee grounds would be OK and put in the trash but I would definitely dispose of them. The reason to put in coffee grounds or cat litter is because, it’s least likely if the medication if found for someone to take it.