Yeah, the theory is clear. In an ideal world, we should have all buildings and infrastructure done by the book. Unfortunately, reality is not always like that. In the particular (imperfect) case, I am just trying to do whatever is possible (and reasonable) without rebuilding the building.
BTW, I finally received a reply from support which answers my actual question:
There is a groove in the grommet, which you can cut to get the cable through.
That explains how to use the GESP with the ATL.
I suppose one GESP right under the ATL and grounded to the mast should suffice for such a short distance (~15 m Ethernet cable).
What do you think?