pips sorry, but your question started "does anyone take the medicine Gabentin" and I don´t.
One doesnt get "immune" to pain killers as such, but I do know what you mean.
I have to presume that your sciatica has been investigated e.g MRI scan and that has, or dismissed the possibility of a prolapsed intervertebral disc.
In direct answer to your question, it is not uncommon for the effects of pain killers to wear off, but there are other alternatives.
I would suggest you see your GP