The issue with this thread is that we're not likely to be able to discern the truly appropriate dosage for you no matter how hard we try. You know yourself much better than any of us. It's more than likely that you've simply become tolerant to either the Gabapentin (Neurontin), the Codeine or both. You can, of course, take a break in an attempt to reset your tolerance a little bit or you can increase your dosage. You're not dealing with anything extremely dangerous here. Codeine is fairly mild and use of Gabapentin in conjunction with Opioids is generally approved of, if not encourage with the hopes of reducing overall Opioid dosage.
So, sadly, you're going to need to figure out what works best for you my friend. We're here to keep you safe and make sure shit does not hit the fan.