Gabapentin potency over an extended period?

The shelf life is 3 to 5 years depending on how you store the medication.

Perhaps after the 5 year term of taking this medication you may have become tolerant to Gabapentin, it happens with many medications after long term use, antidepressants, benzodiazepines, etc..

I would say that if the med has expired and you are taking 1 or whatever period of time after the expiration date many medication lose their potency and efficacy, some may even become toxic like eye drops, liquid antibiotics, etc.

I also take medications and in my experience, such has been the case.

"Expired medicines can be risky
Expired medical products can be less effective or risky due to a change in chemical composition or a decrease in strength. Certain expired medications are at risk of bacterial growth and sub-potent antibiotics can fail to treat infections, leading to more serious illnesses and antibiotic resistance. Once the expiration date has passed there is no guarantee that the medicine will be safe and effective. If your medicine has expired, do not use it.

According to the Drug Enforcement Administration (DEA), many people don’t know how to properly clean out their medicine cabinets. Failing to safely dispose of old medications, especially opioids, all too often leads to dangerous drugs ending up in the wrong hands and can injure children and pets if taken by mistake. The CDC reports that in 2020, there were an estimated 36,564 emergency department visits among children aged 5 years and under for unsupervised medication exposures.

For all these reasons, proper disposal of unneeded medicines is essential."

Source: FDA/DEA