pcb spark gaps
Just be aware that the spark gaps are a bit slow to limit the voltage enough without some other secondary protection. When that is said, I have good experience with spark gaps on the PCB with 6 mils gap between points. They help, by taking away a great deal of energy from the ESD discharge.
PCB layout is VERY critical, not only the sharpness of points and low impedance grounding (to correct grond, which can take the discharge current!). Also how the gap is connected to the signal is important. The signal wire must essentially go via the gap, no stubs. Otherwise they do not work too well. The high speed pulse likes traveling in a straight line, so I have sometimes formed the track so that the gap is in the sharp end of a "v" corner. That maximises the energy going to the gap itself. Of course, on HF signals the added inductance of such V corner might be unwanted, but for low speed/frequency it is OK.