Where to put a spark gap for ESD protection--to an isolated "GND" or to protective earth?
Typically (in my admittedly non-exhaustive experience), protective earth (the line earth) is connected to the chassis, and spark gaps go between device I/O and chassis ground.

This keeps ESD as far away (electrically) from the rest of your circuit as possible. ESD to chassis is by far the least damaging, as ESD won't (hopefully) mess with your circuit, and won't bounce the ground around.

I presume you have a metal chassis?