The change from sitting to standing would normally cause an increase in pulse. I don't think you should be concerned. I doubt gabapentin has even the slightest thing to do with it. Don't let the pulse oximeter cause you undue anxiety.

The PCS brain can easily become over-reactive to changes in demands on the heart. This is part of the anxiety malfunction, not gabapentin. A term used is adrenal-cortical hyperfunction but for most, it is just a minor over-reaction.