If you take a mower in that will not start the first thing they do is tighten up the gap and it will usually start right up.
Gap is decided by the type and strength of the ignition you have.
Over the years I have never seen any gain from larger gap, ever. No gain you can see on dyno or at the track. Get the gap too big and you can see a decrease in performance AKA worn out plugs.
An MSD does not care, it will run with the plug wire hooked to anything.
Just another area where there is no real world gain but plenty of ideas, discussion, worry, etc
.035 will run on anything, .040 will too, many real racers know that you take them out of the box, look for a gap, screw them in and forget about it.
FWIW I use about .016 on my Vertex magneto so it will fire at the plug and not upstream somewhere, that thing will re-arrange your hair-do it is so strong.