K.C.
You can buy pieces of wood and lash them using zip ties to the banister. Then, you drill the gate into the extra wood, rather than into the banister itself. It protects the banister but still provides the safety of a hardware mounted gate.
As for a gate itself, I'm not sure what kind you'll like. We have a safety first gate that we are happy with, but we do have the space to keep it open when not in use. We could keep it closed all the time though if we chose too. My son is five and has been able to open and close it himself for at least a year.