Had to split a gabapentin til pharm opens. took half powder from broke open capsule. stomach burning, nausea, diarehha all day. could this be why?

No: The capsules dissolve in the stomach to release their medication. The medication from an opened capsule arrives in the stomach without its capsule. There is no difference. The only exception would be time release capsules which are composed of small microspheres designed to dissolve over different lengths of time. In taking only part of the contents you may not get immediate results.