Do you know/remember from when you mounted the table whether the GP-Factor backing plate is drilled out behind this "bumper hole"?
Okay, answering my own question. I found an installation video of a GP-Factor table that showed the backside and it's pretty clear, that the bumper gets a drilled (or probably laser cut) space.
This is from a Grenadier install video, but it shows that the bumper gets space in the backing plate. So far so good. The rivets in the screenshot above are from the back, I can only guess this is for space reasons. We have more than enough space behind the backing plate, so will rivet this from the front.