When taking Gabapentin for pain it can take as long as 1 to 2 weeks for the Gabapentin to work on the pain.
The Gabapentin does not work immediately on the pain but instead works over a period of days before it begins getting rid of the pain.
So you'll need to take the Gabapentin for at least 1 to 2 weeks before you start seeing the results of the Gabapentin making the pain go away.
However you might notice that some of the pain is going away partially during the first 24 hours to 48 hours of taking the Gabapentin.
If the Gabapentin is not working on the pain within 1 to 2 weeks then speak with the doctor as it should be working within that time.
Don't make more Gabapentin unless directed by a doctor though.