Can you take gabapentin if you are allergic to penicilin?

Gabapentin is not a sulfa drug, therefore your reactions to sulfa would not be related if you experience an allergic reaction.

And, yes. Gabapentin (Neurontin) does cause weight gain for a great many users. Pregabalin, (Lyrica) not so much. Ask your Dr. if it might be a fit for you.