Missing a dose of gabapentin
would not cause 'breakthrough' symptoms, and gabapentin
doesn't prevent anything. If it works, it just puts a damper on nerve pain, which can get worse as the med wears off.
Of course, everyone is different, but still, I'm betting that the itching is just a (horribly annoying) coincidence. If it keeps up, try asking your neuro for a stronger dose, since at present you're at only half of the max safe level per day.
I hope you're feeling better.