Pre-gapping is the practice of introducing a small, controlled gap between the tilter plates before making fine tilt adjustments. This is done by temporarily inserting thin spacers (such as guitar picks or thin plastic shims) during initial setup. A stiff material that you can pull without damaging the surface will be fine. A pre-gap of 0.5 to 0.9 mm may be all that's needed.
The purpose of pre-gapping is to allow the adjustment screws to work in both directions, pushing and pulling. Without a pre-gap, the plates start in full contact, which means the screws can only push the plate away. That’s perfectly acceptable if the required correction happens to be in that direction, but it limits your range of adjustment.
By introducing a small initial gap, you give yourself a symmetric adjustment range. This generally makes dialing in tilt easier, more predictable, and less iterative, especially when correcting tilt maps that require movement in multiple directions.
Pre-gapping is not strictly required, but it can be advantageous in many setups.
Here is a video of another OGMA product, showing how to create a pre-gap: https://youtu.be/MYEZp_HA5aI?si=YgFbpltwTqogcjl6&t=281