I have an M, and although yours might be different, I found that the location of the slot cut out of the mast track was much too high above the boom for the gate to be really useful.
Sure, it prevented the mainsail slugs from coming out and making a clumsy loose bundle out of the sail, but I still had to remove it (and slide the slugs, one by one, past the opening and back into the lower part of the track) in order to flake or roll the mainsail around the boom and put the mainsail cover on.
I ended up using a small piece of scrap aluminum (was an endcut from a door threshold) over the opening held in place by 3 drilled and tapped 10-32 stainless steel screws. Not a difficult project, even if you’re not handy, and the few tools you do need are cheap, if you have none. I did curve it slightly in a vise to match the outside radius of the mast so it looks like a factory part, and, while it does increase its strength, it's not essential, and really only necessary for aesthetics.
If a sail slug doesn’t happen to align with the cut opening in the mast, aluminum thin enough to be cut with tin snips would even do the job, as the normal pressure put on it would be slight.
The plate and 3 screws easily come loose at the beginning and end of each season when I remove the sail for storage.
For pictures of this see the mods section.