An analogue is not necessarily an agonist.

In fact the Benzos used to be called GABA agonists, but they are now considered more as allied agonists.

Gabapentin was initially put on the market when at that time its mechanism of action was "not known".
http://www.rxlist.com/neurontin-drug...armacology.htm
.

Benzodiazepines: Xanax:
http://en.wikipedia.org/wiki/Alprazolam
So while gabapentin was marketed to be a useful seizure control drug, it is not often used for that... it is used for chronic pain and allodynia. The Lyrica cousin was thought to be useful for anxiety but that didn't turn out to be the case. Lyrica however was given a IV (FOUR) controlled class rating, just in case.

Benzos are useful for the postictyl state following a seizure (usually Valium --diazepam is used). They help to prevent further cascading ones from happening and causing status epilepticus:
http://www.ncemi.org/cse/cse0104.htm

So the Benzos share the antiseizure actions for people that other antiseizure drugs do. Mostly they are add ons and not primary.